A test cross between a pea plant

In pea plants, round peas (R) are dominant to wrinkled peas (r).  a test cross between a pea plant with wrinkled peas (genotype rr) and a plant of unknown genotype that has round peas. going to end up with three plants, all which have round peas. From this data, can tell if the parent plant is homozygous dominant or heterozygous?