The following happened a couple days ago during a competitive adult amateur match.
I was lead AR. An attacker A1 held the ball directly in front of me. The second to last defender D1 was charging hard at A1 (and me), but missed the tackle since A1 neatly passed the ball toward the half line. D1, because of his momentum and slick conditions, slid off the field, behind me and out of my field of view.
In a bang-bang play, the ball was then played forward by a second attacker A2 to a third attacker A3 who was running up the field. At the moment of the pass toward the goal, A3 was clearly in front of any other defenders that I could see, and almost in line with me. I could not tell — because D1 was somewhere behind me now — whether A3 was in an offside position
Question: How do I decide if this is an offside offense when I am unsure of the position of the off the field defender? Note that because of the speed of play, I did not have time to back up, turn around or otherwise judge the location of D1.
USSF answer (November 26, 2008):
If you cannot be sure of the position of the defender, then you cannot call offside. You might consider looking to the referee for assistance. (He or she should be aware of where you are most of the time.) If the referee cannot help, then there is doubt. If there is doubt, then there is no offside — and this applies to both the offside position and the offside infringement.