Is there an assay for yeast to see if they are haploid or diploid?

arent only sperm and egg haploid?

That's irrelevant for yeast, which reproduces through mitosis.

Not exactly. A yeast is defined as a unicellular fungi in a liquid or moist habitat, no matter what family/group it belongs to. Yeasts are not a monophyletic group, more of an ecological one. For the majority of the time, most fungi are haploid, and reproduce asexually by forming spores, or in yeast's case, budding. However they do have the ability to reproduce sexually, and when they do, they form fruiting bodies that are diploid or dikaryotic (two different haploid nuclei in each cell that have not yet fused together).

 

However, most yeast, especially commercial yeast, never get to that stage because they don't need to. They're provided with all the resources they need and never experience the stress that usually drives them to sexual reproduction, and therefore the production of spores that can disperse to newer, hopefully more favorable, areas.