Leo,
If the water is still and murky, there's bound to be bullfrogs. Water lily, duckweed, and algae atop the surface are pretty good signs that you've found a frogging hole. Not to mention, I do more listening than looking. The course croak of an evening bull stands out pretty well against the sounds of smaller frogs that sing through the night.
Biggest bull I've ever taken, by far. I've taken loads with bow, but they were mostly 6 inch adolescents. And man, did he ever taste GOOD!!!