If you pick up the latest Sears Tool catalog, there is a table with the functions checked off. Look in the duty cycle column or row, I don't remember which, and look for the cheapest one with the duty cycle function. If you can't get a catalog for some reason, email me, and I'll look up the number for you.
Donald is correct, if you have an old dwell meter, you can use it for this purpose. Dwell is merely duty cycle measured in degrees of rotation. The middle of the scale is 50% duty cycle. I actually used a 30 year old dwell meter for my lambda adjustment, then didn't trust it and spent the $30 bucks for the Sears DVM. I put it on and it agreed with the cheap, 30 year old dwell meter.