Finding Charge of a battery without knowledge of prior charge
If you haven't been counting coulombs, You can only roughly estimate the state of charge of a battery from its terminal behavior.  That's why we count coulombs.  

The best way is to stimulate it with low and high current draw (or wait for your equipment to do that) and estimate its internal resistance -- but even for that, for most useful cell chemistries the whole range from 80% charged down to 20% charged or so there's not a lot of difference, and there are differences from cell to cell and as the cells age.

(Edit):

You can sort of make a distinction between "full to burstin'", "somewhere in the middle", and "about to poop out" -- but that middle section will account for most of the discharge cycle of the battery.