The best advice I can say is that you'll know.
I'm am asked that by just about every on snow class I teach for Friends of Berthoud. Since everyone is new to the game, I put a beacon in my pack, bury it, and each person to doing a basic beacon search. Not one of them had a problem telling that they hit my pack with a probe. There is a just a different feel to it. Part of it is that the normal depth you are probing too, is a lot more shallow. If you hit a large rock, you can feel it, so when you hit something softer like a full pack or a victim, you just feel the difference.