If you're ordering online, order from TE. They're great about returns.
Still, while trying on a pair of shorts can eliminate some shorts, and sitting on your saddle with them on can eliminate more, there's no way to really know whether a pair of shorts will work for you besides just riding in them. Yup, that can get expensive. That's why a lot of us, once we find one pair that really works for us, will buy several pairs...