Clowns are easy to identify, they have a pattern all their own that looks different from normals.
The dorsal stripe, the small spots along the sides, and some other markers that aren't as easy for me to put into words but you pick up on looking at clown pics online are things to look for.
Hets will look like normal wild type ball pythons, since it is a recessive gene.
There's no point IMO to buying a het unless you buy a pair of snakes both carrying the gene, which you plan to raise up and breed. Since you haven't stated your intentions of whether or not you're looking for just a pet or a breeder, keep that in mind.