When in doubt, you can always run your stealth character around casting farsight (or the equivalent) to see behind walls. Even knowing the arrow indicator, I always have Tami go around the map casting farsight where there are large unexplained black areas.
Still haven't found how to spot them. I even sent the party around trying to step on one to see if I could see the tile difference, if any. It makes sense if there's not a visual clue, they are under water after all but man, stepping on one at the wrong time ads a level of frustration fighting lizards.