Why can you tell the genotype of a trait inherited by the intermediate dominance pattern?

Incomplete dominance produces a "blending" of the two alleles. With incomplete dominance, heterozygotes will always show an intermediate phenotype. For instance, taken the example of red or white snapdragons. The flower colour exhibits incomplete dominance, where heterozygotes are pink. If they are pink, we know the must have one red allele and one white allele. If they are red, we know they must have two red alleles, as if they had a white allele they would be pink instead of red. Likewise, we know that white snapdragons must have two white alleles, because if they had a red one they would be pink instead of white!