It is almost impossible to accurately know the value of charge stored in a battery. The amount of charge a battery can hold varies with battery life, battery use, charging method and environmental issues.
Niel's explains nicely how to know if the battery is fully charged (with liquid electrolyte filled batteries) and Tim's tracking of the battery charge state by watching the battery voltage, and the current, likewise.
However, both are only indicators of how much charge is in the battery. Not of "How much charge the battery can hold" and, more to the point how long it will take to discharge.
Battery Lifetime Characteristics of Lead Acid Batteries
Over time, battery capacity degrades due to sulfation of the battery and shedding of active material. The degradation of battery capacity depends most strongly on the interrelationship between the following parameters:
the charging/discharging regime which the battery has experienced
the DOD of the battery over its life
its exposure to prolonged periods of low discharge
the average temperature of the battery over its lifetime
Measuring voltage is pointless as this will decrease as soon as load is applied. Even a nearly useless lead acid battery can, under certain circumstances, indicate a high voltage.