You can download a smartphone app called ‘gates carbon drive’ designed for checking belt tension. You could also use a guitar tuner app.
JPDiag software also has a harmonic tension tool but would need a windows laptop with an external microphone, so is a bit more faffing around.
There’s also some info here on other methods (scroll down for
http://www.ducatisuite.org/belttension.html
Sorry, i don’t know what the ’spring tension’ equivalent would be.