I just started getting flashbacks of past lives from a really young age and then when I was old enough I researched all the facts and found out most (well, the facts that I could find) were correct.

So I assume that memories coupled with physical/historical proof would definitely point to or affirm a past life.

Other than that I suppose simply "knowing" certain skills, like a talent, and feeling like you've done it so many times before would be an indication.

Also meeting other people who have the same historical interests as you who you swear you have already met but cant remember when is another hint.

People who share exactly the same memories/dreams of another life as you and, as a result, are able to recognize you from the "dream" (memory) when they meet you in true life.

There's also tendencies to be able to understand another language as a young child and speak it fluently.....without ever literally learning it if you know what I mean.

Also often kids who are freakishly smart who go on about how they want to go back "home" and how they used to live *insert name of planet/city/description of historical house...etc* should probably be taken into account.

I'm only picking things off the top of my head from personal experience here, so there's probably a lot of other ways to tell if you've had a past life and what the past life was.