Only lifeguards need to be able to spot it at 50'. And they'll get training.
For everyone else: you only need to spot the silence. People in water make noise and move. If anyone isn't doing that, talk to them. You don't have to say "hey, are you drowning?" just "hi" or strike up some small talk.
If they aren't responding and don't seem to be doing anything but bobbing along with their mouth barely above water, that continued silence is what should urge you into further action.
