Sean Hester , Mar 12, 1998; 10:32 p.m.
the easiest way to determine the fresh ones would be to buy some fresh ones, measure them, then compare them to the other ones. any that are not exactly the same voltage as the new ones are suspect.
the problem you have is that the voltage of a dry cell battery varies wildly with the load being put ont he battery. meaning that a new battery might measure 1.7 on a volt meter. (a volt meter (by design) doesn't put a load on the thing it's measuring) but when you put it in a flashlight, turn the flashlight on, and measure the voltage then, you'd only get 1.5 volts. say you then put a used battery to the same test. it may measure 1.6 (or even 1.7) on a voltmeter. but in the flashlight (when it's actually being asked to supply power) it may only measure 0.9 volts.
if you want a short answer, i'd say no less than 1.5 volts on a voltmeter. that may sound silly, but it's probably true.