This article http://www.bikexprt.com/bicycle/tension.htm
explains the theory and gives a table of frequencies for various spoke lengths and types.

I built a wheel using this method and it worked pretty well. I found a musical pitch program online that would tell you the frequency of a tone via your computer microphone. I held my mike next to my wheel as I plucked the spokes and just read the frequency on the screen (yeah, I know, I do have a good ear, but this way was much more high tech).