The wraps-per-inch (WPI) people post for handspun fiber is typically on the finished (plied, or singles if you chose not to ply) yarn.
That said, measuring the WPI on the singles gives you a reasonable estimate for the thickness of the plied yarn; typically, a 3-ply is twice the thickness (or, half the WPI) of the singles -- consider the singles are all wrapping around each other in the plying, so you won't get 3* the thickness). And depending on how you spun and how tightly you plied, the 2-ply is usually in the neighborhood of 2/3 * the WPI of the singles.
I did math on a Mabel Ross table (Essentials of Handspinning, most likely) of singles-vs.-final-plied-WPI once, and came up with numbers from 55% to 60+%, and I think Anne Field also discusses thickness of singles-vs-final-plied-WPI in Spinning Wool: Beyond the Basics. The two rules of thumb above have stood me in good measure (think PUN) for most of my spinning (caveat: I knit what I spin, so it's usually not very tightly spun...)
(posted by me this day on spindlers)