title: How to check if user paid on android?
body: <p>I would like to use in-app billing in my android application (instead of creation 2 versions of application - free and pro). If user paid, then additional options in Preferences should be available.
  My application synchronizes data with website (not my). 
  Each time synchronization happens, I would like to check if user paid or not. 
  How should I do it?</p>