I have had several discussions with referees about the proper procedure for an AR when a throw-in is either illegal or improper (never enters the field of play).

A player for the attacking team was taking a throw-in and stepped on the field. I immediately raised the flag straight up in my left hand and waited for the refereeā€™s acknowledgment. Once the whistle blew, I pointed for a throw-in for the defending team. The center referee told me that I should have waved the flag. I argued that if I waved the flag, that I would be providing information that I observed a foul. I could not find this specific issue in the Guide to Procedures, but I reasoned that it should be treated similarly to a ball that leaves the field and immediately returns and is still being played.

If a throw-in never enters the field of play, I normally signal for a throw-in for the same team. When I am the referee, I normally tell my ARs to follow this procedure.

Thanks for your help.

USSF answer (October 12, 2010):
The Guide to Procedures is clear: The assistant referee “Supervises throw-in elements per pre-game conference” (p. 18).

That means that the AR should keep the referee informed if the ball is not thrown in accordance with the procedure outlined in Law 15 or never enters the field. This, however, should be discussed in the pregame conference and the AR should not signal at all if the referee has a clear view of the situation.

Note that any AR involvement in signaling problems with a throw-in should be ONLY within the terms of what the referee wants done, as discussed in the pregame. If the referee does not make clear what, if anything, the AR should do in the case of any illegality in performing the throw-in, ASK.

And, assuming the referee has directed the AR to signal certain violations of Law 15, the correct signal is for the AR to raise the flag straight up, make eye contact with the referee, and then signal the correct restart (e.g., for an illegal throw-in by Red, give the throw-in signal in favor of Blue).