Quote - ...So did he make it or not? I have no way of knowing. I removed it from my scene and am making a building of my own.

MacMyers is right. There is not so much you can do if you only want to use original objects. If you find a car model somewhere, you essentially do not know where it comes from. You can search with google for "car model" and look at the first three million images to check if they look similar, but if you do not find something, it will still be unclear if yours is a genuine model or not.
As a result, most people (like me) are not willing to spend too much time on checking the source of the models, and will just go ahead and use it. Legally you are relatively save when it comes to using models with unknown origin, i.e. you are not obliged to make sure you are not buying/downloading stolen stuff, except in the most obvious cases (when the descripton says "ripped from ...."), so it is basically a question of how much time your good conscience is worth to you. If you want to make money with it, things can get more complicated, though.
If you do not want to spend so much time, but have buckloads of money, there are lawyers/private investigators/registration agencies (i do not exactly know how they are called), who specialize in those things. Perhaps you can also try to hire MacMyers for it :-)

Quote - ... But it seems I shall have to learn to make cars. I just can't buy all the cars I need!

This is probably the main reason why professionals tend to make everything themselves. Essentially this is the safest way to only use models you are allowed to. And even then, if making models of real existing cars or objects, they are walking on thin ice.