Your eyes are your first weapon. Still water doesn’t move—it’s the couch potato of the liquid world. If you see ripples from a breeze, a fish, or a child throwing a rock, that’s moving water, and it’s showing off.
Still water is like a mirror that’s given up on life. It’s perfectly flat, sometimes with a creepy, oily sheen that looks like a unicorn’s bad hair day.
Look for scum. If the surface has a greenish, brownish, or rainbow-colored film that doesn’t move when you yell at it, congratulations—you’ve found still water. Algae and bacteria are throwing a slow-motion rave on top.