The best marker needs to be DARK and reasonably opaque - and easy to remove. Black white board markers are best, black wax crayons, chinagraph pencils and indelible markers running a close second.
(see my next Instructable on resurecting white boarrd markers)
It works like this:
The rim is rubbed with a dish washing abrasive pad and some kind of solvent such as kerosine, turpentine, acetone, dish washing soapy water etc... making it more or less oil free and clean.
An ink ring is applied to the side of the rim, by placing the marker on the side of the rim, and rotating the wheel slowly.
Then the wheel is roatated really fast, some kind of friction material - usually wood, is slowly brought towards the rim, until the high spots JUST start to rub against the wood, and rub the ink off.
Bingo! There are all your high spots where the ink is rubbed off, and the low spots are where the ink still remains.
The beauty of this system lies in it's absolute simplicity, and the combination of 4 feed back systems.
You can VISUALLY see the high and low spots RECORDED, where they occur, on the rim.
You can FEEL the high and low spots, as the dowel runs over them.
You can see the GAP between the dowel and the rim.
You can hear the rubbing of the dowel on the rims high spots.
And as you dial the wheel in - by very fine adjustments of the spoke tension - tightening some spokes, and loosening others - in 1/8th, 1/4 to 1/3rd turn increments - you can dial the wheel in absolutely true - to easily within about 0.2mm or 0.3mm - because this is incredibly tactile, and you can SEE and FEEL and HEAR the dowel and the WAY it rubs on the rim.
When the rim is out of true, the dowel rubs against the rim, with a "silence" where it runs above the low area, and a "ssssttttt" where it rubs on the high spot. So as the wheel spins it makes a "silence" - "ssssttttt" - "silence" - "ssssttttt" -"silence" - "ssssttttt" -"silence" - "ssssttttt".
And when the rim is running perfectly true, the noise of the dragging dowel is "Ssssssssssssssssssssssssssssssssssssss" with no real variation in tone or breaks in the tone.
And you can FEEL it - the tactile feed back. And you can SEE how the rim runs closer / far away / closer / far away - from the dowel or if it runs parallel.
This is so fiendlishly simple and brilliant to use.
Step 1. Push the marker pen against the rim and rotate the wheel slowly.
One pen width and one revolution seems enough.