How can I tell if my seedling is male or female without flowering it?

The most common method is to force flower a labeled clone to determine the sex, rather than the incorrect method of forcing the seedling and then reverting it to vegetative growth again.

If you are a seedling grower who does not clone, or a grower who has no seperate vegetative and flowering grow spaces, the easiest method is simply to wait for the preflowers (primordia), and sexual maturity to appear while the seedling is still in vegetative growth. In other words, if you wait until they are 6 to 8 weeks old, most plants will show you their gender without any forcing.

Seedling growers who don't clone are really missing out on the full potency potential of their finished crop whenever they force their seedlings before they've become sexually mature.