Pea plants naturally self fertilize. So a pea plant that hasn't been manipulated will be "true breeding" that is, homozygous. You can also tell after you've let them self. The homozygous ones have all the offspring look the same, the selfing heterozygotes have a quarter of their offspring look different.

Pea plants naturally self-fertilize. So a pea plant that hasn't been manipulated will be "true breeding" that is, homozygous for the allele in question. You can therefore tell after you've let them self-fertilize.

Homozygous individuals have identical offspring for the trait when selfed, whereas selfing heterozygotes have a quarter of their offspring looking different for the trait.

In other words, you can't tell in the present generation who is heterozygous without molecular methods unavailable to Mendel. But if you are willing to self the individuals, save seeds, plant them, and track where they came from, you can figure out who was heterozygous/homozygous in the last generation.