How did Mendel understand which plant is true breeding?Becoz all the plants would look alike i guess?Tall plant is tall only but how did he know whether it is pure tall or not?

Dear student,

Mendel first crossed the parental generation one with tall phenotype (TT or Tt) and another with the dwarf (tt). He got the hybrids in F1 generation. Then for checking the purity and genotype of the tall parent plant, he performed the test cross in F1 hybrids.
Test cross - The crossing of recessive homozygous with the parental dominant one to check whether the dominant character was homozygous or heterozygous for its character.

Regards.