How do you know if a snake is het?

I plan on breeding but I still do not understand how to know if a snake is het and what % its het.

find out what the parents are...

And if the outcome is that they aren't sure it is a het than you can proof that it is or isn't by breeding it and see what the baby's look like.

Agreed. Without knowing what the parents are, you won't know for sure what hets your Corn has.

There's been some discussion of "visual markers" for certain hets, but these are very much the minority.

Well what I mean is if I had a pair of Hypo 100% het for plasma and the babies came out plasma what Het would they have?

Plasma is a mix of lavender and diffused. If the offspring were plasmas, they'd actually be hypo plasmas because the parents are both hypo. And... that's what they'd be. No hets because they'd be homozygous for every trait the parents are carrying in heterozygous form.

Ok so if there where any normals would they be het? and one more question if I breed my het plasma female with a plasma male will I get plasma babies?

As both hypothetical parents are hypos, then all their babies would at least be homozygous for hypo. Any "normal" hypo offspring would have a chance at carrying the lavender and the diffused gene in heterozygous form. Each parent would have a 50% chance of passing that particular mutant gene to the offspring, so there would be no 100% hets. 100% means that the baby is absolutely carrying a particular trait in heterozygous form.
If you pulled this out into a punnet square, you'd find that about 2/3 of the babies would be carrying the diffused gene in het form and another 2/3 would be carrying the lavender gene in het form. And there would be a small overlap between the two that would include babies carrying *both* genes in het form. Because of this 2/3 ratio that occurs when the parents both are het for the same trait, this becomes a 66% het. There is a 66% chance that the baby is carrying that particular trait... but it also means there is a 33% chance it is *not*.
If only one parent has a particular het, then all the offspring have a 50% chance of carrying that trait.

If you breed a hypo het plasma (diffused lavender) to an actual plasma you will get some plasma offspring, yes. The plasma parent will automatically contribute a diffused gene and a lavender gene. Your het girl would have a 50% chance of contributing a particular gene. So 50% of the babies would be lavenders. 50% would be diffused. And there would be an overlap where both genes are present. So in the end you'd have a clutch that was 25% normals het for hypo (from mom), diffused and lavender (from dad), 25% lavenders, 25% diffused, and 25% plasmas (this is the overlap. Half the diffused and half the lavenders carry both traits together, equaling a plasma).

Thanks I understand a little bit of it now. I think I will understand it all in 3 years when mine is breeding age

Understanding genetics is often very confusing to some people. Often, seeing the same thing written differently by several people helps. Have you read the stickies at the top of this subforum that discuss genetics?