Another request leading here at Seeds Aside… Sexing plant.
Unlike animals, plants do not have a defined germ-line (and thus are not ‘weismannian’ organisms). So for plants that have retained sex oddities in their evolutionary history (that is, not being hermaphroditic like most other plant species), there’s usually no way to sex them before flowering. (The question is of course useless if you were thinking of ordinary hermaphroditic plants, unlike this one for example).
Unless the sexes also differ in architecture or other features (leaf shape and so on). In this case you can sex your plant some times before they decide to bloom. I think this is the case for Cannabis sativa, which is usually the plant species some internautes of the student kind care to sex. But even so, you can’t sex it at the seed stage. And I don’t know of any plant that be sexed at such early developmental stage.