There’s an app called “Tensioner” that does an OK job of calculating spoke tension based on plucked tone. As Doug Fattic mentions, spoke diameter and free length from nipple to first crossing are input variables. Both are easily accomplished with “dials” on the iPad. Just be sure that ambient noise is very low. It’s results are reasonably close to the fancy DT spoke tension meter that I was recently gifted.

Pre-internet, I’d just compare the plucked tones to a known good wheel. When instructing fellow volunteers at Bike Works, where we encounter bikes that have suffered all manner of abuse/neglect, I tell them to shoot for a middle-of-the-scale musical note.