You can't! Measure specific gravity at the beginning when most of the juice is still in the fruit, that is. And of course if you leave a mesh bag of fruit in the fermenting must for several days, then squeeze out the juice when it's easy to do, which is my method, any measurement taken then will be meaningless.
So... I've found it helpful to start with a recipe that gives amounts of fruit, sugar and water to produce an end result of x%. There are formulas available that you can use to adjust the sugar amount to get a different strength wine.
Alternatively you can calculate starting water amount (if using any) and starting juice amount, look up sg of the fruit juice, do a weighted average, then calculate how much sugar to add for your desired starting sg. If you plan on squeezing the heck out of the fruit, expect the pulp to be 15-25% by weight of what you started with. Blueberries have lots of pulp, peaches less.
Take lots of notes for future reference!