Would you bother preparing something to help prove your reincarnation, or would you simply take advantage of whatever you retained (and prepared for yourself) from your past life?
How might you use this ability to help yourself and others?
Edit:
To clarify, suppose you have some vague memory of your past life in your reincarnated self that helps you along and allows you to take advantage of whatever your past self prepared for you.
I'd work throughout my current life to make the world a better, kinder, healthier, more sustainable place.
Which I'm doing anyway, because it's our duty to future generations, whether or not they are our reincarnated selves.