You can figure out how much twist singles yarn need if you decide to 2-ply them later, even if you've already set the twist in them.
First, with a short piece and a sink of warm water:
Take an 8" length your single that has had the twist set in it, fold it on itself and knot it together at the open end, and put it in a sink of warm water. "Milk" the folded yarn strand along its length so that as the yarn gyrates, it twists around itself along its length (plying in the warm water, basically). The warm water re-activates the twist in the yarn, twisting it along its length to show you how much plying twist this yarn wants to have a balanced 2-ply. Save this sample and compare your plying to it as you spin for a balanced ply.
Second, "just by looking" ... as you ply, look very closely at the singles. The plies are balanced when the fibers all run parallel to the length of the yarn. It takes _really_good_light_, really good eyes, or a magnifying glass to check this closely. If the yarn has some variegation, that can help, as you are looking for the curly twists of color to straighten out along the length once it is plied.
Note that your plied yarn will appear to be way over plied -- don't let that worry you! Just wash and set the twist as you did with the singles, and it should all come out balanced as long as you've matched up to your samples or watched the straightening of the singles' fiber as you went.
(from a post by me on spinningfiber/livejournal)