In theory, it is not possible to prove that an animal is not heterozygous for any given gene. In a test cross like A/- x a/a, if higher number of the progeny is with dominant phenotype, then it is less likely to say that the parent is heterozygous. For the cases that the parent is heterozygous, half of the progeny should be a/a and the other half should be A/a.
Therefore, if n number of progeny are with dominant phenotype, then we can say that the probability that the parent is A/a is (1/2)n.