You may be able to with certain strains/phenos but most I've run won't show from seeds for several weeks which is difficult to keep * in good shape * long enough to sex them in a cup. I just had one that I was able to sex but I kept it in the cup for like 7 weeks from seed and it looks like shit, will take a week or two to get it growing properly again. That was just a matter of space and time but I normally wouldn't do this. I just up-pot to 1gal and I can always sex them in there and keep even one's that take forever to sex in the 1gal's (fabric/smart pots) without getting root bound. If you do anything to force sex them do it with a clone. When you throw any plant into flowering, you can revert it but you'll lose weeks and end up with mutant growth.