If the relative clause is between comas it is non-difining relative clause.
Example: Sarah, who is my best friend, has a dog.
If not, it is a defining relative clause.
Example: The man that is standing over there is my father.
That is a simple way of recognizing them.When there are proper names you do not need defining relative clauses because names are already defining. You can also check whether you can drop the relative clause or not, if you can drop it, it is non difining. So if you need the RC (you cannot drop it) it is defining.
In the first example we can drop the RC; Sarah has a dog. we don`t need the RC because we know who we are talking about, the fact that she is your best friend is irrelevant.
But in the second example we cannot drop the RC: The man is my father. What man? The man that is standing over there. If we drop thr RC we don`t know who we are talking about