The purpose of the Way of Adidam is God-Realization. The Way of Adidam "works" because it is based on the Divine Person incarnating here in the human form of Adi Da, thereby making the Divine State available here and now, in every moment. The Way "works" by devotees accessing the Divine State via their devotional and spiritual response to Adi Da.
God has always been present. There has never not been a moment when the Divine has not been present, because the Divine is our very State and Source. The entire universe is arising in the Divine. That having been said, however, one of the primary communications of Adidam is that the always present Divine is not accessible to us except through the appearance of a human incarnation of the Divine. We don't know how to locate the Divine in the abstract. We do know how to locate another human being, and it is possible to locate the Divine through a human being who is transparently revealing the Divine.
The core principle behind the Way of Adidam is: You become what you meditate on. If you want to Realize God, then you must locate God and meditate on God. But how to do that? The Way of Adidam provides the means. Because Adi Da is the Divine here in person, His appearance here has provided an easily accessible means to locate and meditate on God.
So to "test" the Way of Adidam, you must become a devotee, and meditate on the Divine State, using the human form of Adi Da as the "gateway" or "touchstone" to locating that State. But to become a devotee, you must first receive the Revelation that Adi Da is the incarnation of the Divine. The Way of Adidam is not based on the belief that Adi Da is an incarnation of the Divine, but the Revelation that this is so. Much more about that here.
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You can certainly "peel off" secondary pieces of the Way of Adidam, test them by themselves, and find them useful in their own right. For example, the vegetarian (and mostly if not entirely raw) diet recommended by Adi Da certainly is useful in its own right, for promoting health, longevity, and even greater emotional equilibrium, even if you are not purposed to God-Realization. As Adi Da puts it:
Also: since the reason it is useful for God-Realization is because it helps free up energy and attention (cooked food, junk food, etc. all bind attention to the body, stultify energy, toxify the body, make one ill, negatively impact one's emotional states, etc.), then it is useful for anything less than God-Realization that can benefit from freeing up energy and attention: anything from winning an Olympic Gold medal, to working more effectively, to attaining lesser Spiritual Realizations than God-Realization (like becoming directly aware that there is a greater-than-material Reality, whether through psychism, awareness of astral dimensions, etc.).
You can also look at excerpts from Adi Da's Teaching that include observations or statements about the nature of Reality — there are countless such observations! — and compare these observations against what you have discovered or concluded about the nature of Reality. In this restricted sense, you can "test" the Teaching of Adi Da (if not the Way itself).
But the purpose of the Way of Adidam per se is God-Realization, and neither the secondary practices associated with Adidam nor study of Adi Da's Teachings bring about God-Realization. Only becoming a devotee of Adi Da is the means for God-Realization in Adidam. So the true test of the Way Itself can only occur after becoming His devotee.
Thus, the first real "proof" of the Way of Adidam is the one that enables you to become a devotee in the first place: the receiving of the Revelation of recognition of Adi Da as the Divine Person. The further testing of all the different details of the Way can only be done in the context of that initial and ongoing recognition of Adi Da — since the point is to progressively deepen one's immersion in the Divine State that Adi Da is, over time, to the point where one Realizes that Divine State oneself. All the various disciplines and other aspects of the Way of Adidam are about that progressive deepening, and that is what one "tests" over time. One sees how taking up these disciplines actually does make it increasingly easier to turn to Adi Da in every moment (and re-immerse oneself in that Divine State), actually does deepen one's immersion in the Divine State, and actually does progressively weaken one's (otherwise automatic) egoic recoil from that Divine State.
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Because the Way of Adidam is all about cultivating and deepening the devotional and spiritual relationship with Adi Da (based on recognition of Him as incarnation of the Divine Person, our own Native State), in some sense it is a like a marriage. And to speak of "testing" the Way of Adidam makes sense only to the degree to which the notion of "testing" a marriage makes sense. First: you can't "test" a marriage unless and until you get married! Second, any test of a marriage is as much a test of oneself as it is of one's spouse.
But a marriage isn't fundamentally about "testing". It's about falling in love. It doesn't make sense to talk about testing a marriage if the "falling in love" part hasn't happened. In the context of a marriage (or a potential marriage), the only purpose to talking about "testing" (or "trial marriages", or living together first) is to just to get a sense of whether this relationship is going to endure, weathering the ups and downs of daily life, beyond the initial attraction and the initial expression of love.
But that's where the analogy between "becoming a devotee" and "getting married" breaks down. "Falling in love" is analogous to receiving the Revelation that Adi Da is the Divine Person. But once you get that Revelation, you also instantly realize that the only limit on the Revalation and the relationship is you, not the Divine Person, into Whose Person and State you are surrendering. God is that which is Perfect, Unlimited, Eternal, and Perfectly Happy. Ego is the limit. This "marriage" is all about surrendering a limited identity into the One Identity, the Divine Person. It is not a relationship or marriage in the ordinary sense — a marriage of "equals". It is a total surrender of "self", the lesser identity, the mortal, suffering identity, for the sake of Realizing the Perfect, Unlimited Identity. The "test" is always only on one side of the relationship! Am "I" going to surrender "me" in any moment. . . or not.
Once the primary commitment to that relationship is in place, there are many secondary details of practice that one must explore and experiment with. One tests the various alternatives and determines which form of practice works best in one's own case.
OK, so you are interested in or intrigued by the Way of Adidam; or you are moved by Adi Da's Wisdom; or you feel His Spiritual Transmission in some way — but you have not yet received the Revelation that He is the Divine Person. What can you do?
1. Get help — You don't have to rely solely on your own resources. This site is full of firsthand accounts of Who Adi Da is, and about the Way of Adidam. In addition, you can get in touch with any of the devotees who have received this Revelation. Get them to tell you their story!
2. Immerse yourself in Adi Da — This response is meant not to discourage you, but rather to clarify, inspire and move you to do what is required to receive this extraordinary Revelation, this miracle which many otherwise ordinary people have received, by doing the necessary preparation. Your best bet is to immerse yourself in the contemplation of Adi Da: His Word (books, websites, audio talks, etc.), His Image (photos, videos, etc.), His Art, serving His World Work, etc. That Revelation will be Given to you by Grace, sooner or later — guaranteed!
3. Grow in self-understanding — Because this Revelation is not just a matter of immersing oneself in Adi Da, but also a matter of self-understanding — of discovering (and transcending) all the ways one is pre-disposed to (unconsciously) block the reception of that Gift — I'd recommend considering which of the following "pre-Adidam" issues (not specific to Adi Da or the Way of Adidam) may apply (to some degree) in your own case:
If any of these patterns are in play in you, they will tend to block the reception of Adi Da's Revelation, either limiting it or shutting it down altogether — sometimes through knee-jerk reactions. (In fact, these tendencies and viewpoints are just as much a matter of consideration for devotees as for interested non-devotees, because they represent the habits of a lifetime; they don't just disappear overnight, even after the Revelation is given, and one becomes a devotee.) For instance, if you are bothered by the "non-egalitarianism" in a phrase like, "Only-By-Me-Given" (which appears here and there in Adi Da's Teaching, and is based on His comprehensive, comparative study of all the precedents in the world's spiritual and religious traditions), that very disturbance will tend block to your reception of His Revelation. You'll be so caught up in your reaction to His words, that you'll miss Him!