Singlespeed & Fixed Gear - how do i guage my chainline?

as the title suggests... how do i tell if my chainline is right on? i've tried using sheldon's directions but don't think my measurements are correct. the reason i ask is i've been fighting this grinding/squeaking sensation coming from my drive side crank for a while, and after trying everything i know of i've come to the conclusion my chainline must be off. it LOOKS okay, but i'm no expert, so how can i tell for sure?

thanks folks...

Sure fire way: get something long and straight like a ruler, dowel, or whatnot. Lay it flat against the chainring so that it extends backwards in the same plane to the cog. Does it match up with the cog or do they lie out of plane with one another?

genius! pure genius! *sigh*

I've actually taken to eye-balling mine, with decent results, I might ad. Before that I measured my axle, divided that in half and used that measurement to mark the center of my rear hub, then measured out to the chain. Then I'd measure from the center of the seat tube to the chain (much easier) then compare. Again, this wasn't too scientific, but got me close enough. Also, I read on the forum here that someone places the bike on it's side, with the frame level, then simply checks the chain with a level. I seem to remember he used a stand somehow. But I imagine supporting it with blocks or whatever would work, as long as you're able to adjust everything to render the frame level. A novel technique, I think I'll try it myself. DanO

I've got a piece of hardwood toungue and groove flooring about 2.5 ft long and the groove fits over the sprocket and the cog-a small ruler does the comparison.

Eyeing in my experience is difficult, my chainline looked pretty much perfect but when using a ruler it was 4 mm off. The reason I measured was the noice from the drivetrain which almost disappeared when I corrected the chainline.