Often the first hint about a past life people have is an inexplicable interest in a particular country, language, time period, historical event, etc. And often when presented with photographs or movie footage which related to their past life, they might feel an emotional rush and even identify themselves somehow with what they are seeing.
For example, an English TV documentary showed a American who had become so fascinated with the American Civil War that he had turned his house into a Civil War museum. Not surprisingly, under hypnosis he remembered being a soldier in the Civil War. This can be viewed by the sceptic as wishful thinking, but conversely it follows that a Civil War veteran would always have a fascination for that period - probably even into their next incarnation.
And an example of these obsessions with history which often seem irrational to the outsider are the current proliferation of military re-enactment groups all over the world, covering formations as diverse as medieval knights, through to soldiers of the American Civil War, WW1 and WW2.