Extremely few hets carry a visual indicator, especially for recessive traits. The only way you can be certain you are buying an animal het for a certain trait, is reputation of the breeder/person you are buying from.
At a reptile show, go for the well established breeder, talk to them about any guarantees etc. should it turn out the het(s) you bought don't produce the morph in question after a couple clutches. Ask to see the parents, often they bring the adults to a show to show what they look like, though not always. Don't be afraid to pay a little extra from a big breeder, than somebody selling hets for a unusually low price. Though not all 'bargains' are going to be a scam or ripoff, some can be.
There is said to be a genetic marker for het pieds but from everything I have read upon it, its a more hit or miss sign, which to me, doesn't really provide a guarantee. Both het pieds and normals carry the marker and many het pieds don't. Thus, not much of a reliable indicator in my book.
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PHLdyPayne