<p>Is there a good method or tool to level the bed of 3D printers? I find myself making small adjustments a lot and it's mostly just trial-and-error. A normal bubble level is of limited help and trying to figure out if the head is the thickness of a sheet of paper from the bed in all corners is beyond the capability of my vision.</p>

The easiest way I know of (unless your printer has a Z-probe and automatic leveling), is to bring the nozzle(s) down fairly close to the bed (maybe 1/4" or so), and then move it around while watching for anyplace that doesn't look even. Adjust the bed until it seems even. You can just eyeball it, or use a ruler or object to measure.
Then bring the nozzle down closer, and repeat. Each time you move it closer, you'll be able to eyeball more accurately.
Once you're quite close, pull out a 3x5 card, or business card, or similar, and move the nozzle up or down until the card just fits between the nozzle and the bed (with no great pressure, but no space either). Again, move the head all around the bed, and do any remaining (tiny) adjustments so it's the same everywhere.
Of course if the bed is at all warped this will be much harder, or even impossible. So before starting, put a good-quality straightedge along it to make sure it's actually flat.