How did Mendel know if a plant was a homozygous tall (TT), or a heterozygous tall (Tt)? I had some issues while studying the Mendel’s rules of inheritance. How did Mendel, while running his test crosses, know if the plants in his F0 generation were TT (homozygous tall) or Tt (heterozygous tall)? There seems no way to differentiate between them, and I do not know of any sophisticated tools which may do so, but surely they were not available to him? There may be some flaws in my logic, as I am a beginner, but I am not able to find any sources which tackle this problem. I am familiar with the basics of inheritance.

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**.