soilsampler had it right with the plumb bob.
tie a piece of string to your stem with a few inches hanging down. make sure the knot is in the center of the stem, then tie something small (a mailbox key works well) to the piece hanging down. you'll know you're lined up right if its pointing directly at the center of your tire.
even easier is to hang a necklace from your stem. its not a crucifix, its an alignment compass!