Checking the wing frequency on a glider always seems to be fiddly - you need to be able to count beats and seconds at the same time, and depending on who you listen to you should count for a minute, or 30 seconds, or something else. With this app all you need do is rest your phone or tablet on the wingtip, press Start, then rock the wingtip up and down. The app will monitor the resonant frequency and when it is sufficiently stable give you the answer! Typically this takes 15 seconds or so, and it will be accurate to within +/- 1 beat per second.
For consistent results you should perform the test under the same conditions - i.e. glider in cradle vs. on main wheel. There will be a difference between the two readings so do it the same way at each inspection.