How to tell if my phone has been hacked/monitored ?

My phone was stolen a while back, and mysteriously reappeared. Now it seems as if a close "friend" always knows what I'm doing and who I'm hanging out with I installed lookout l, and it doesn't say my phone has been jail broken, an no strange apps show up in a spotlight search. But how can I be sure that my phone isn't being monitored or been hacked into without resetting it? And is a complete reset going to definitely wipe it clean of everything? Thanks!