It's very hard to tell with solids. If your eyes are good enough you might be able to tell from the weave. It's flatter on the back and more raised on the front.
A hint for keeping it consistent throughout your quilt, if you decide one side is the "right" side, mark the back side every few inches with a chalk pencil before cutting. Then you won't have to decide each time you pick up a piece. This works great for batiks too. :)