[A Snake]

There should be a visible ID tag and a frame stamp. Have the seller show you both, then call the manufacturer (if they're still in business) and verify.

Not to sound snobbish, but if you begin looking at "bargain cars", you'll likely find the company no longer around.

Get some really good pictures of an original and use them to CAREFULLY compare your prospect with.
The closer the car comes to an original as to body lines,dimensions, chassis and accessories, the more likely it is to be a well recognized piece....and still either be in production or well enough recognized to be verifiable. I have a Contemporary, which is now no longer made, but is still well recognized.

Don't be fooled by trick paint or wild crap under the hood. Yes, you can certainly have these and get a good car too, but sometimes, that's all there is...trick paint and a monster under the hood. The rest can be pretty hokey.

This club is a really great place to ask owners about their cars. If you see something you like, put up a post and ask these guys. Virtually all of the better replicas are represented in this membership and these folks know their cars inside and out.