I wouldn't have thought of using flourescent tubes as straigtedges. Not a bad idea (if you happen to have some)Ludwig wins the prize!! Another way is to use two 8-foot fluorescent light bulbs. If your fenders are removed, you can stand the bulbs straight up gently touching the rims or tires. Two friends can gently hold them there or, if you don't have two friends then use duct tape. Now step back and eyeball the bulbs to see if they are in the same plane. (Parallel with each other.) This method can also be used to adjust the tension in the head steady if you have an adjustable one.
Just remember that the bulbs being parallel (in a vertical plane) does not indicate wheel off set. You can, however, run one bulb under the bike and see if your tires or rims are parallel when you hold the steering straight. When the front rim is parallel to the rear rim the distance from the straight edge to the front rim or tire should be half the width difference between your front and rear tire/rim.