What kind of cross might tell you if a black guinea pig is bb or bb?

Genetic cross
   This is the reproducing of two distinct people that result that carries properties and traits that conveys a bit of the hereditary material of both the parent people. The parent that are involved in the cross breeding process might be from species that are firmly related or from various assortments and the parent people are perfect hereditarily.

Well, you can use test cross to find if the individual have a dominant allele or not, if it is dominant and homologous it will produce new odfsorings with dominant allels, if it is dominant and hybdrid it will produce 4 individuals 2 of them are dominant hybrid and the other are resasive