There aren't any guaranteed ways to tell visually that he's het clown (not a "clown mix" - het means he is carrying the clown gene). If the breeder is reliable, and knows the pairing that produced him, then the het is as good as his word.

You get het clowns by pairing a visual clown to a non-visual clown:
Male:
Female:
Normal
Percent Fraction Traits
100% 1/1 Het Clown