Is it rechageable or not? A "dry battery" or "dry cell" is normally not rechargeable.
In either case, it is difficult to estimate the energy remaining in a battery. To do a reasonable job you really need a smart device that can monitor current into the battery (for a rechargeable battery), current out of the battery, terminal voltage, and probably temperature. There are ICs designed to do this - try googling Benchmarq (I think they've been bought out by Texas Instruments). Microchip do this sort of thing too I think. Google "battery energy monitoring circuit" or something like that.
If the battery is rechargeable, you may already need smart circuitry for controlling the charging process; this circuitry may also be able to monitor the current drawn from the battery and estimate its condition.
Also, as Steve suggested, you may be able to get a fairly approximate indication of charge level by looking at the terminal voltage, provided that you either know the load on the battery or can temporarily remove it, since the terminal voltage is affected by the load on the battery at the time you measure it.