To get a more accurate estimate of the pitch, you measure over multiple thread pitches and divide by that number. So if you have 6 threads, measure the distance from crest to crest, and divide by 6-1 (5) to get the pitch. If there are N crests total, the distance is (N-1)*pitch from the first crest.
A thread gauge is handy if you machine stuff. As long as the thread isn't too fine, any low cost thread gauge will do. If you need to measure fine threads, I'd recommend you pony up the cash for a better thread gauge. A cheap gauge is ~ $10. Good ones are ~ $60. Typically a cheap gauge only measures one kind of thread, say SAE, or metric, or British Whitworth. So it is worth measuring a few threads to get an idea what thread system you are dealing with. If your measurement is very close to a multiple of a metric pitch, it's likely to be metric (with a couple of exceptions). (The pitch on an M5 screw, 0.8mm, is real close to 32TPI, but they don't quite fit. 0.8mm = 0.03145", 32TPI pitch = 0.03125".)
