Referees and especially Assistant Referees should be familiar with the basic requirements of offside (Law 11) before each match.
The most basic element, and the only one covered here, is that in order for there to be an offside violation, an attacking player must be closer to the opponent’s goal than either the second to last defender or the ball (whichever is closest to the goal line) AT THE TIME THE BALL IS LAST PLAYED OR TOUCHED BY A TEAMMATE. If a player is in an offside position and comes back onside to receive a pass from a teammate, that is an offside violation. If a player is NOT in an offside position, she can run on to a through ball and receive the pass closer to the goal than the second to last defender, and there is no violation. This is why it is so important for the AR to dynamically adjust position in order to be even with the offside line – the second to last defender or the ball, whichever is closest to the goal line.
Offside/Deflections:
If an attacking player is in an offside position when the ball is last played or touched by a teammate, and that player then plays the ball after a deflection off a defender or a goalkeeper save and parry, that is an offside violation. ARs and CRs should know this rule, but if a goal results from such a play, and there is uncertainty, at a minimum the AR should stand still on the touch line and not jog up the line toward the halfway line to indicate a good goal. If the AR stands still, the CR should come over to the AR to confer to determine what happened and whether there was an offside violation and the goal should not be allowed. Even if the CR has given the signal for a good goal, that decision can be reversed up until the time of the next restart.
Offside/Confidence:
ARs should know Law 11 and be dynamic in their positioning to ensure a good perspective to see offside violations. One trick that I have used when AR is to keep a running tally in your head of which players on the attacking team are in an offside position and when the offside slate is clear. With the basics of knowing Law 11 and proper positioning, you then need to see the play and make the call by raising your flag. The CR is depending on you to do so, and neither the CR nor anyone else will be in a better position to make the call. It is your call to make