How to Sight a Chain Line I just finished building a single speed bike and I think my chain line might be a little off.  Is there a tool or trick to sighting the chain line?
You should be able to "sight" this fairly well.  Get close to the front sprocket and align your eye such that you're just looking down one side of the front sprocket towards the rear.  Moving your head back and forth sideways very slightly the same side of the rear sprocket should flicker into and out of view.

Or you can use a long metal ruler, placed against the side of the sprockets.  With perfect alignment (which isn't really necessary) the ruler will fit flat against both sprockets simultaneously.