To find out if no-one has any mines left ("Mine" for west mines, "MineE" for east mines).
({ "Mine" in (magazines _x) } count (units (group player))) == 0;
You could use this in a trigger condition or in a waitUntil block. One possible issue with this is that if someone dies then they wouldn't be in the group any more and so whether they'd placed a mine or not wouldn't be counted (This checks that all living members of the group are not carrying mines). With this technique, one also has the problem that it makes no distinction between dropped and placed mines. An alternative would be to count the number of mines on the ground with a nearestObjects check, as I think placed mines are "Mine" and mines that are dropped from inventory are seen as "WeaponHolder" or something (can't check that right now, though).