Does it tell us exactly who/what, etc? Without it would we be able to identify the person, thing? If it tells us who or what and we couldn't identify the person, thing, etc, without it, it's defining. If it's just throwing in some extra information, then it's non-defining.
The man who stole you car is sitting at tyhe table next to us.
Here, 'who stole you car'makes it clear which person the speaker is referring to- it is essential information and is defining the person so the listener can tell who is being talked about.