A smarter thing to do would be to not hire them in the first place.
How would an employer know? Somewhere in the 0.25% - 0.5% of the population is transgender, but it's way, way, rarer than that that you see someone in public and can tell that they're trans. Most trans people blend in seamlessly. If you work for a large company, there's a good chance you have (or have had) a trans coworker and never know. Any time you're in a crowded restaurant, mall, stadium, etc., there's a good chance you're around a trans person without realizing it.