Some good Clown pictures here: http://www.ballpython777.com/clown/clown.html

both colour and pattern differ from a normal-phase BP. That said, Clown is recessive so a het clown will look like a normal. To get a Clown, breed a het Clown to either a Clown or to another het Clown. If bred to a Clown, your het Clown will produce Clowns about half the time...het to het would be a one-in-four chance.

I'm not sure there's any way to prove a het anything really IS a het anything without breeding it, so...you have to trust the seller. A het Clown, like any recessive morph, will look like any normal.