All of the above, I think. There are so many ways it's hard to put a finger on a 'definitive' how.

Usually the best place is to start within yourself. Sometimes an almost-obsession with a person or place in history or somewhere you've never been to before but you still feel the connection with. It's all about the connections, so if you feel one (with a person or place) then it's a good place to start. Also remember that we are incarnated with the same Souls - Soul Group - so there's another place to look at. Chances are the people close to you now were close to you before. A sister in a Past Life may be a mother in this or a close friend, but if you look behind the mask of the physical body and try and see the Soul inside you may be surprised at what you find.

Another thing is feeling as though you're not yourself. Now and again you might get the feeling that although you're right here right now, you may feel as though you should be someone or somewhere else. Sometimes the feeling of "I wonder what it would have been like to live in those times or be that person" is a hint. Or you may feel as though the person you've just met has been around you all your Life. Why don't you simply ask them? Just be a little careful how you broach the subject in case they think you're a total whack-job :-) Start with your own instincts and see how you get on from there, but use your discernment. Sometimes it's easy to get caught up in the fantasy of it all so be as honest and as objective as you can be.