How can you tell if someone is heterozygous or homozygous for a trait that is dominant

By crossing it with recessive homozygous

Explanation:

If you cross dominant homozygous with recessive homozygous all of the offspring will be heterzygous and express the dominant trait. If you cross heterozygous with recessive homozygous, half of them will have dominant trait expressed, while other half will have recessive trait expressed.