How do I find out the make and model on a used ATV with no VIN number?

I bought a used Yamaha ATV. There is no VIN number. How do I find out what year and model it is?

I only found numbers on the engine. ThanksHow do I find out the make and model on a used ATV with no VIN number?
There are at least two numbers on motorcycles and ATV's. One on the engine and one on the frame. I have several Yamaha ATV'S and the number on the frame is on one of the tubes that is towards the bottom where the engine is. Unless the frame has been power coated or modified, it should be there.

It should also be on your title or paperwork if you have them.