Usually not worth it; you'll need to gently distill (in proper glass, this is important) the beer like it is done for ABV measurement and measure density of leftovers. Or do quantitative chromatography. You've got to know hands-on chemistry real well for this. Let me know if you need the procedure.
It will be only rough measurement, you can just take FG in °Pt (Bx) and it is %of sugar so pretty easy to calculate it.
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Some starting points for beginners:
Quick and diry guide to fermenting fruit - cider and wine