In general, a person's performance in their job is the best evidence for their future performance, followed by tests you can give them, followed by their genes. That's not to say that there aren't pointy haired bosses who could be sold a load of snake oil on the subject but that's probably nowhere you'd want to work anyways. And with medicine pre-existing conditions are a much worse problem than genetics could ever be but thankfully in the US at least our existing laws seem to have that in hand.