Well you can't be sure, unless by some odd chance you knew the thief. But most times anyone breaking into a car is looking for a quick buck. The resourceful ones will have tools in order to pry a head unit out of the dash. The rest won't think twice about smashing your window just to see if something might be under the seat.

When something like this happens to you, you become suspicious, and very analytical. For example, when my previous car was broken into almost 2 years ago, I was convinced that the thief was on foot, and possibly lived or had friends that lived nearby. I was home and awake at the time, and no vehicles passed by my house between the time I parked my car and the time I found it with the window smashed.