To determine if a particular plant is homozygous or heterozygous, you would have to test cross with a

In order to figure out if an individual (for a specific gene) is heterozygous or homozygous dominant (because the phenotypes are the same) you'd test cross it with a homozygous recessive.

If all the offspring have the dominant phenotype, then ur plant (individual) is homo dominant. But if there are any offspring with recessive phenotype (specifically 50% of them) then ur plant is heterozygous.