It's when you notice the white hairs turn orange prematurely. If you have a hermie then there is a good chance you will notice this by the 3rd week into flowering. Bud development will also appear to slow down as the plant shifts its energy into producing the seeds. That being said, sometimes they may not be pollenated until later giving you a harvest that is full of tiny little undeveloped seeds. This will give it a woody taste and smell and also a crackling noise when you smoke it.