If the plant has recessive genotype then it will be easy to identify because it will have the recessive phenotype. However if the plant has dominant phenotype it can either have homozygous genotype or heterozygous genotype. To confirm if the plant is homozygous or heterozygous, a test cross can be done with plant having recessive phenotype.
If our test plant is homozygous dominant, it will pass one dominant allele to the next generation and all the offspring would have dominant phenotype. If our test plant is heterozygous dominant it will pass one dominant allele to half of the offspring and one recessive allele to another half so 50% of next generation will have dominant phenotype and other 50% will have recessive phenotype.
Hence by test cross (cross with recessive phenotype plant) it is possible to determine the genotype of the uncharacterized pea plant.
