The demon of uncertainty haunts the field--and the people in it. For this reason, more than one researcher has offered criteria of credibility, or universally-applicable guidelines for how much to believe a given story, based on common sense as well as their experience. I’ve compiled a list below that draws from several sources. Not just famous past-life claims but any past-life memories can be roughly evaluated this way.
1.Verifications. Knowledge from memories which is confirmed by a source the person has not accessed is by far the best evidence. The more of it can be independently verified, the better for the claim.
2. Seriousness. People who treat reincarnation and past-life memories as a parlor game as not likely to have experienced true memories. If they aren’t aware that these experiences are about the very core of who they are, and don’t seem to be carrying any emotional baggage, it’s fantasy. People who have showed their commitment by having done a lot of work, especially healing work with professionals, are more likely genuine.
3. Humility. If there’s anything that should teach a person that all people are innately equal, it’s being in an obscure life after having a prominent one. People who boast, take a self-important tone or condescend are probably just manifesting the key diagnostic criterion of Narcissistic Personality Disorder: “Fantasies of and preoccupied with beauty, brilliance, ideal love, power, or unlimited success.”
4. Honesty. If someone lies about something else, their reports of past-life experiences are not trustworthy.
5. Consistency. A story that keeps changing is the result of disorganized fantasizing, not internal fact-finding, since facts do not alter over time, though they may be clarified or added to. In her books on children’s past lives, Carol Bowman gives four signs to distinguish past-life memory from the typical fantasizing of children, and this is one of them. But some adults do it too.
6. Grip on reality. If the person makes other statements that are preposterous, his past-life claims are to be dismissed. Disorganized thinking is a fair eliminator also. The more well-adjusted and grounded a person is, the more they’ve come to terms with their demons, the more credible he is. People who live in reality know that others want evidence of such claims, and don’t mind being asked.
7. Similarity/continuity. Every soul has traits which carry over from one life to the next. They can be anything: appearance, style of dress, preferences, mannerisms, handwriting, interests, personality traits, worldviews, health problems, emotional issues, or what have you. The more of them there are, and the more specific, precise and idiosyncratic they are, the more credible the claim. Specificity and idiosyncrasy are particularly important; it means nothing if a person claims a personality trait that is shared with a large portion of the population as well as a historical figure. In some cases, especially with personality, a person can have a polar opposite trait, due to overcompensating in this life for a previous one.
8. Long-term fascination/aversion to the time period. A person who has had a long-term fascination, especially a strong one, with a particular period or region in the past is more credible subsequently claiming a past life in that period/region. Conversely, a longstanding distaste for a period in history or historical figure can be the sign that a person was connected to it in a past life in a way that he or she wants to avoid remembering. The aversion factor is very good for separating out fantasizers, as they tend to hold their claimed incarnation as a cherished possession, rather than something to avoid.
9. Childhood memories. If the person talked about a past life or depicted it in drawings from very early in life, that suggests a true past life.
10. Groups. A claimant who has found other people who were associated with him or her in the famous past life, who fulfill these criteria themselves, do much to solidify a claim – especially if they independently verify memories with their own memories. However, all members of the group should reasonably satisfy all other criteria.
11. Memories. You’d think this would be unnecessary to state as a criterion, as no one would claim a past life without having any memories of it; but some do. The memories should:
a) not contradict any firmly-established historical facts (with the caution that many historical “facts” are not known certainly at all, and the “accepted story” can change); and should fit in with -- and even illuminate -- the conditions and culture of the time.
b) show a balance of good and bad, joyful and painful. An overly-romanticized past, or an overly-demonized one, is not a credible one.
c) include material not known to history, filling in some of the backstory behind known events, and giving credible answers to questions that historians have given up on.
d) include intensely emotionally-charged events
e) have the “ring of truth” about them – creating a gut feeling of reality.
12. Self-sourcing. In my view, our past-life truth has to come from inside ourselves; our inner knowing is the only true authority. Thus claims that come from sources other than the person him or herself, such as third-party psychics, channelers or famous-past-life “matchers,” are dismissible. Those arising from sessions with qualified regression therapists are credible since regression therapy is a method of accessing inner knowing.
13. Healing effectiveness. If retrieving the memories and knowledge of a PL has caused a true, noticeable improvement in a person's mental/emotional health, self-esteem or chronic mood, that cannot be explained by some other factor, the PL is more credible. To quote my regression therapist: "You cannot heal by a delusion."