When should I flag for a foul the referee missed?

From the practical guidelines (2026/27):

• Raise the flag when a foul or misconduct is committed in your immediate vicinity or out of the referee's vision. In all other situations, wait and offer an opinion if it is required, then tell the referee what you saw and heard and which players were involved.
• Before signalling, decide that the offence was out of the referee's view (or the view was obstructed) and that the referee would not have applied the advantage.
• Use the "wait and see" technique: let play continue and do not raise the flag when the team fouled will benefit from the advantage. Eye contact with the referee is very important.
• To signal: raise the flag with the same hand you will use for the rest of the signal (it shows who gets the free kick), make eye contact and give a slight wave back and forth, avoiding excessive or aggressive movement.
• Foul by a defender inside the penalty area, out of the referee's vision: first make eye contact to see where the referee is and what action has been taken. If the referee has taken no action, signal with the flag, use the electronic beep signal and visibly move down the touchline towards the corner flag.
• Foul by a defender outside the penalty area (near its boundary): make eye contact and signal with the flag if necessary. Show with a clear movement along the touchline towards the halfway line that the offence was outside the area.
• If you signal for a sending-off offence and the signal is not seen immediately: if play has been stopped the restart may be changed; if play has restarted the referee may still take disciplinary action but not penalise the offence with a free kick or penalty kick.

Source: IFAB Laws of the Game 2026/27, Practical guidelines for match officials, and Law 5 (https://www.theifab.com/laws-of-the-game-documents/).