Just a quick video using an app. I tried gStrings, a guitar tuner but the"Check Spoke" App (here) had better filtering for frequencies and added regression if you know the length, diameter, and material of your spoke.
Haven't checked to see if there is a good iOS equivalent yet, but I'm sure there are guitar tuner apps. Just match the frequency of the spokes, if a spoke it way off you might adjust it down. Keep in mind there is a balance between a true wheel and good spoke tension. If you aren't careful you could mess up how true your wheel is to fix the spoke tension, or more commonly, the spoke tension is messed up to fix the trueness.