It helps if you know the area and the road and what landmarks are around to give you a clue. Signs, poles, bridges etc. can tip you off to how high the water level is. You can pull over to the side of the road grab a stick and poke at it to get a better idea. The water will often be deeper at the edges.

Another common but unreliable tactic is to wait for someone else to try their luck and see how they manage. Some cars and trucks will do better than others. If you do take your chances aim for the middle and go slow. Still water after a storm is dangerous enough (you can't tell what's below the surface) but I'd never take chances with visibly moving water. Even shallow water moving quickly can knock you off your feet or push your car around.

If you have any doubts at all the best thing to do is to turn around and find another route. If you drive in an area long enough you get to know which areas are prone to flooding and which roads are usually safe.