At the low levels, I'll do a lot of early season coaching. "Hey, 14 - are you supposed to be on the line? Step up son ..." and then take 'em aside afterwards, chat with the white cap, offensive captain, coach, whatever might be needed to help 'em get it right. In pee wee games, kids are often playing at the game for the first time, and they need that help ...
At varsity and above, I will only tell a receiver if he's "on" or "off" if they ask or make a gesture to me. I'll do it loud, clear, every time. Never say a player is "good" or "OK" - just whether they are on the line. "I see you as ON", with the latter word slow and loud, so that if I say "I see you as OFF the line," the different word, and the different length of the phrase acts as a trigger.
If I see something amiss, early in the season (six on the line) I might say "86, I see you as OFF the line" after the first statement, just to ensure they realize what I see - but remember, sometimes, illegal formations ain't the fault of the receiver: a guy playing tight end has his head in the clouds, didn't check in, forgot to tell the coach he's injured, his sub didn't report, whatevah ...