I just came up with this and it seemed to work well for me. Hopefully it will help someone else. It is best to have dark sheets on when doing this:
Step 1: Buy a crayola washable marker (the kind you give to kids)
Step 2: Lay on bed
Step 3: It is preferable to have someone else do this, but you can do it to yourself. Angle the marker so that the tip of it is pointed to where your body and your mattress meet. Draw a line up your body from the bottom of your butt to your shoulders, always making sure to keep the marker at the intersection of your body and the sheets.
Step 4: Go check your fine self out in a full length mirror. The line you just drew should follow the normal curvature of your spine.