It's true that male plants tend to grower tall and lankier. This is to facilitate the spreading of their pollen over the females, which tend to be shorter.

However, in this new day and age of cross breeding it could very well be different phenotypes, especially if you have 50/50 indica/sativa. So, like they said, flower them all to make sure.

Males also tend to flower quicker so you should be able to pick out all the males well before the females start full speed flowering.

I keep my quickest flowering male and always use its pollen to make seeds with my best females. Hopefully, in this way I can eventually have a quicker flowering plant.