Backing up a little bit before you understand morph types you should understand genotype.

Genes come in pairs, one from each parent. If the pair of a particular gene is a matched pair it’s the homozygous genotype. If the two copies of a particular gene are different it’s the heterozygous genotype.

A recessive morph or mutation type is a mutation where the heterozygous animals (with one mutant and one normal copy of the gene for that mutation) look normal just like the homozygous normal animals. Only the homozygous mutant (with two copies of the mutant version of the gene – one from each parent) are a visible morph. An example would be albino where the hets are normal looking but the homozygous mutant animals are visible albinos.

A co-dominant (some prefer incomplete dominant and there is a technical difference but I’m not sure which is more correct for the ball python mutations) mutation is one where the heterozygous mutant animals do not look normal. Even one copy of the mutant gene is enough to make a visible mutant. However, with co-dominant, the homozygous mutant genotype produces a visible mutant that looks different than the heterozygous mutant genotype. An example would be Mojave where the hets are Mojave and the homozygous mutants are mostly white snakes.

A completely dominant mutation would be one where even one copy of the mutant gene dominates the normal version and makes the heterozygous genotype just as much the full mutant phenotype (appearance) as the homozygous mutant genotype. If homozygous spiders are indistinguishable from heterozygous spiders except for breeding results then that would be an example of a completely dominant morph.

As far as predicting the outcome of a breeding I believe you should start with the genotype (heterozygous or homozygous) and the basic understanding that each parent randomly gives one of its two copies to each offspring. With that understanding you can see that the only guarantee is with homozygous animals. With those you know what sort of gene they will pass on to each offspring because they only have one version. So even if spider is completely dominant mutation type a heterozygous spider is still rolling the dice as to if each of it’s offspring gets a mutant or a normal version of the gene from it just like with any heterozygous animal regardless of mutation type. All the dominant part tells you is what the heterozygous animals look like in relation to the normals and the homozygous mutants.

So, it’s the genotype heterozygous that tells you each baby has a 50% chance of getting the gene from that parent and the genotype homozygous that tells you that all the offspring will get the gene. The recessive, co-dominant, and completely dominant mutation types just tell you what the hets and homozygous ones look like in relation to each other. Also, those mutation types apply to the mutation and don’t change between heterozygous and a homozygous animal. For example, Mojave is a co-dominant mutation regardless of if you are talking about a heterozygous regular Mojave or a homozygous “super” Mojave. It would be incorrect and mixing descriptions to call the super the “dominant form” of Mojave.

Possible hets are from recessive morph breedings involving heterozygous animal(s) where you could produce both normals and hets but since the mutation is recessive you can’t tell them apart.

If there are homozygous spiders and they look and act just like the heterozygous ones then yes, the breeding results would be how you would identify the first ones. Once you prove a pair you could breed them together and knowing that neither has a normal copy of the spider gene to pass on you would know that all their offspring where homozygous spiders.