The only way to determine the value of a plant for breeding is to grow and examine its progeny. The most potent plant might not pass on this trait, and the healthiest plant might not have favorable
combining ability. Judging a male by visual appearance and also smoke testing, or
using bio-assay, should not be discounted, but test crossing the male is the only way to determine it's breeding potential.
See also
Soul's Selecting Breeding Individuals...