* Head low in the water, mouth at water level
* Head tilted back with mouth open
* Eyes glassy and empty, unable to focus
* Eyes closed
* Hair over forehead or eyes
* Not using legs – Vertical
* Hyperventilating or gasping
* Trying to swim in a particular direction but not making headway
* Trying to roll over on the back
* Ladder climb, rarely out of the water.
People who are actually drowning don't have the resources to thrash around and yell for help. Someone who's thrashing and yelling could well be in enough trouble that they should be helped, though.
Link thanks to Johanness Dahlquist at Less Wrong.