Unless of course it is a homozygous spider! I know, I know, that hasn't been proven, but until it is one way or another we have to be open to the possibility right?
I wonder if the op meant to ask how to tell the difference between a het and a normal??? If so, with recessive traits, its only through breeding. With codominant and dominant, there are visual differences, which is what Tim was saying. Spider is assumed to be a dominant trait, and thus far, no homozygous spiders have been proven. So, it is sometimes assumed that homozygous spiders don't exist.
Steve