Well, if you think your snake might be het hypo, you breed it to a hypo snake. If you think your snake might be het lavender, you breed it to a lavender snake. And soforth, for all the morphs you want to test against.
If you bred it to a hypo snake and some of the babies showed the hypo gene, then the snake is het hypo. If you bred it to a lavender snake and some of the babies showed the lavender gene, then your snake is het lavender. On the other hand, if you breed it to a hypo or a lavender and don't get any hypos or lavenders, respectively, then your snake is not het for those traits.