Dad passed away, how do I figure out what property is still in his name?

So my dad passed away about two years ago and left my mother in charge of his estate.

I have some family telling me that a few of his cars that he lent out to friends before he died are still out there being used.

Is there anyway to find out if he still has vehicles in his name?

The cars could be registered in either AZ, CA, or WI