One should always setup passcode/fingerprint lock as basic security measure on their iPhone or iPad. For some reason or whatever, you might not want to passcode lock your iDevices. If you then suspect someone is snooping on your iPhone, then you’re out of luck as there is no way to determine what the person has done on your iPhone.
Here’s a cheap trick that you might want to try to find out if anyone is messing with your phone. First, follow these steps to remove all your background running apps. You should end up with the home screen as your only “app” in multitasking. Now off your phone and leave it near your suspects.
To check if your phone is being used while you’re away, activate multitasking by double-clicking on the home button. If there is any app shown up in the multitasking bar, then your phone is being snooped.
This trick will not work if person using your phone is savvy enough to delete their tracks by removing background apps. But it should be sufficient enough for the average users.