verifying unique app purchase

I am looking for a way to verify if my app was downloaded by a user. I want to allow users to notify me of their purchase if they intend to donate to a cause and we would donate a percentage after verification of app purchase. I would want to 开发者_StackOverflow社区see/validate a proof of purchase. It has to be unique per app download.

If you don't have a logged in user, you can generate some unique code on every dowload ( it might be even a GUID) and supply it to the user. Then he/she must enter this code on purchase.