It is better to find out before you load it for sure and you machine quilt to the bottom pins or zipper and the backing is shorter than the quilt....DRAT!
This week it happen to me when loading a Veterans Quilt for my Quilt Guild. I have 4 more to go, hopefully they have enough fabric.
After turning it every which way I decided to add an extra 12" of fabric to the backing length after I squared it up.