if your skis are symmetrical, find an empty corner of floor and put your skis bases flat on the floor, tails against the wall, edges touching at the contact point and one ski with its outside edge on the wall. if the lines are even, that means the top sheets are either fine or off an equal amount, to find out which, flip one ski tip to tail, base still flat on the floor, now one skis tip is at the others tail, if the lines are still even, they are dead center, otherwise, break out the tape and measure those babys up!
or just measure them if my explanation is to confusing, I am not very good at explaining things