People can't really tell. I would say you can be safest by assuming all visible flooding is too high, especially if you can't clearly see road markings.

A lot of people do monkey-see-monkey-do: observing other people driving through water and then trying to follow. Some people just go slowly until it feels too sketchy and then try to back up.

People inevitably get stuck.

The really big issue is when the road is lower in some spot and you don't expect it.

For example, in my city there is a road that will be perfectly clear until you hit a small section that's a low spot at an underpass. Cars driving too fast hit that section during a heavy rain and quickly get flooded/stranded.