Hi, Lied! Welcome to "Gotmead
Good question (with a somewhat involved answer)!!
With fruit that hasn't been pressed into juice, you can only estimate the amount of sugars available for fermentation. I've covered this subject in a past thread; if you use the search tool and search for the terms 'fermentable sugar in fruit' you'll find, among other things, this:
My posting (I think it is around #10 in the thread) explains how to estimate the fruit's contribution to your total sugar content in a melomel must. If you've got any questions after reading the entire thread (ignore the puns), then let us know!