How to find out if someone is lying about stealing and using yor iphone?

My iphone 4s gone missing for one day and the other day the app find my iphone turned on and I found it at my friends house. So is it possible to find out if he was doing something on my phone, if he turned on or turned off, when ther was no sim card inserted

We were at a party and somehow it gone missing, and the next day it occurred at my friends house and he denies that he stole it. No it does not have a passcode

Does you phone have a passcode? If yes, then unless he guessed it, he did nothing with it.

Were you at your friend's house? Could you have lost it there?

If the 'friend' denies it and you haven't been there to misplace it -- it's a police matter and you need to file a theft report. Then call your cell provider and do the same

Completely wipe it down and don't handle it with your bare hands. Then see if fingerprints appear on it.

Don't do anything. Your phone was returned. You don't know if he really stole your phone.