Stuming a spoke tells you nothing. You need to strum a bunch of them. And you don't need to be a pro. Begin somewhere and pluck one and then the next, and the next. Some will sound very similar and one here or there will be noticeably higher or lower. Keep plucking in the area where you will little by little tighten or loosen the offending spokes and adjust the nearby ones so they don't get to out of "tune". Then go back to your truing and stress relieving routine. Repeat.