applications - How do I know when an app was used in Android?

Is there any way to tell when an app was being used in an Android phone?

I am specifically interested in the camera and gallery functions. I believe my roommate went through my photos without asking, and I've tried to view the cache for the Gallery app but can't seem to figure out how to do it.

Is this even on the right track? He had it for a twenty minute interval, and I want to see if the Gallery was accessed during that period of time.