Dear student,
If the plant is phenotypically dominant for a particular trait, to find out experimentally whether an individual plant is homozygous or heterozygous dominant for that particular trait, a special type of cross is performed which is called the test cross.
In a test cross, the plant under investigation is crossed with a homozygous recessive plant for that trait and the phenotypic ratio of the first generation offsprings is observed. If the cross presents 100% dominant trait then the plant under observation is homozygous dominant. And if the cross presents 1:1 ratio of dominant vs recessive traits in the offsprings then plant under observation is heterozygous dominant.
And obviously if the plant under observation is of recessive trait, then obviously it is of homozygous character.
Regards.