If your guitar will not stay in tune, the pattern of the drift tells you why. A string that goes flat after bending is usually slipping at the tuning post or the bridge. A string that goes sharp after a bend is catching in the nut and not sliding back. Strings that creep flat over several minutes are often new and still stretching, and a slow steady change in every string can be the weather moving the neck.
This test measures those patterns instead of guessing. First tune up and take a starting reading of every string. Then bend each string hard a few times, use the whammy bar if you have one, and play each string again without retuning. After that the test waits and checks again every few minutes, beeping when it is time, for as many rounds as you choose.
Each reading only counts once the note holds steady, and strings can be played in any order. At the end you get a chart of how far each string moved, a table in cents, the drift speed of each string in cents per minute, and a plain-language note for every string that moved.
The last result stays saved on this device, so you can close the page, make a fix and run the test again to compare. Results can be saved as a PDF.