How do you know a car is a prototype?

Title. What separates a prototype from a custom? How do you know for certain it's one and not the other? I've been collecting for about 5 or 6 years now and I still can't quite make heads or tails of it.

I've also seen cars that just looked like completely normal unspun errors sell for booku money on ebay just because the seller calls it a prototype. What am I missing?

With all the prototypes being sold and traded on these forums, I'd be a little surprised if no one could answer any of these questions... :)

Seems to be a well kept secret.

No not a secret, i know in the past there have been several posts which if you do some searching the questions will be answered, sometimes cars will skip a step like paint or having the posts spun or no interior and will be called a proto, but sometimes those are legit pre production cars, as far as what makes a redline a prototype? I am no expert so someone like sid, bruce pascal, or turbofire could answer those questions better than i can, they are top notch guys and generally willing to answer questions,
Blank bases (can be faked) and barbie plastic are generally sure fire bets, also sometime you run across paint samples that have digital numbers on the roof are out there, they come in blister packs believe it or not, i found one at a target

Matt,
great questions, I know there are many more items for sale as "prototypes" than there could possibly be. :D
I do not own any myself.

As far as the Redline prototype history, a great resource is the book [I]Hot Wheels Prototypes, written by Bruce Pascal & Michael Zarnock[/I].

It's available at on-line retailers and is very insightful.