What would you cross a round pea with to determine if it were heterozygous or homozygous dominate

It will be crossed with a homozygous recessive i.e wrinkled pea plant.

Explanation:

This test is known as Test Cross which is used to identify the homozygous or heterozygous dominant genotypes.

If the cross gives all round pea plants then the parent round pea was Homozygous Dominant.

If the cross gives 50% or even any quantity (even 1) of wrinkled pea plant then the parent round pea was Heterozygous Dominant.