Did it have any plates on it? Maybe you can trace the VIN
backwards from there.
In most western states you can't just take an abandoned trailer and get it licensed. In CA you would have to file a mechanics lien against it (for parking fees?) and post notices etc. Then once the lein is approved the DMV will create a new VIN plate for it if none is available.
But to answer your question, they are often stamped into the frame in the tongue area. With a lot of paint
on top it might be hard to find.
I think that the first thing to do is contact your local DMV office for advice on how to proceed.