I don't confess to be good at doing it all the time, but I do try to listen for audio cues.
If I'm top of a wall for maps such as Bluefin Depot, Moray Towers or Flounder Heights, and I start hearing sounds that I can't visually find sources for, then I need to get away from the wall or else I'll get ambushed by whoever's coming up. I don't play with team chat, but even if I did my allies would have to be facing the wall from the enemy side or be watching sideways at the wall in order to warn me that someone's coming up, both of which are quite unlikely.
I can aurally differentiate between rollers, chargers, bamboozlers, blasters, splatlings and shooters. I can sometimes tell if something is a 96 gal, but the gals in general I tend to get mixed up with shooters. Usually if I know that whatever I'm facing has shorter range than I do, I back up and use my range advantage to take them out. Otherwise I'll hide to the side and try to attack from behind.