I think mailing in a sample for genetic testing is the only way to find out before they show preflowers. Some plants can show preflowers in veg without a light switch, some people wait for plants to do this before flipping.
There may be some people who have a knack, or some autistic spectrum where this is possible, but I bet if those same people bought a $300 pack of regs to grow flower they would not take a hammer to the "male" ones before germination.