I think they determine offsides as and when the ball is being passed to a player and whether the player is in line with the defense or not? For instance, two strikers makes a run through the backline of the defense. The ball is passed to one striker A on the left side exactly when he is in line with the defense. According to the rule, this is legitimate. Because he has not crossed the defense line or any last defense player before the ball was passed to him, he is ok. But the other striker B made the similar run. Before the ball was passed to player A, player B had already crossed the last defense. Now at this circumstances, player A is onside, but player B is offside. The lines men will allow this. Why? Because player B has not yet involved himself in active play. So player A, if he scores, the goal will count. But player A say passes the ball back to player B, assuming he is still in front of the defesive back, he will be offside. Slightly complicated, but undestandable it seems. So the main thing here is when the ball has been passed. That "when" usually determines when the player is offside or onside.