I was reading a discussion of plying on Julia's blog and thought I would write a little about plying and twist angle since I couldn't stick a picture in her comments!
When you ply singles that are freshly spun you can hold up a loop of yarn and see if it twists in either direction. If it hangs straight or nearly straight then you have a balanced ply. However, if your singles are even a few minutes old, some of the twist energy has been set. That's because the friction and the scales are sort of interlocking a little bit so it doesn't want to untwist as much as it did when you first made the single. Therefore you can ply that and do the loop test and end up with an unbalanced yarn (usually underplied) because you were testing singles that weren't really "awake". However, when you wet them, the twist energy wakes up and then you end up with unbalanced yarn.
There is a way to test yarn balance however, whether your singles are fresh, old or downright moldy from sitting in your closet for a year! That is to look at the fibers in the singles and see what angle they have in relation to the yarn's longitudinal axis. The individual fibers should parallel the axis of the yarn in a balanced ply. (this is easiest to see if you have a multicolored single, but use a magnifying glass if you can't see the angle). If the fibers instead are slanting in the direction of the ply angle, you have overplied. If they are slanting in the opposite direction to the ply angle, they are underplied. This method works no matter what kind of singles you are using, old, new or a combination of both.