The first and most likely indication of a low battery would be a hard starting problem caused by slow cranking. If the battery seems weak or fails to crank your engine normally, it may be low. To find out, you need to check the battery's "state of charge."
A battery is nothing more than a chemical storage device for holding electrons until they're needed to crank the engine or run the lights or other electrical accessories on your vehicle. Checking the battery's state of charge will tell you how much juice the battery has available for such purposes.
If your battery is low, it needs to be recharged, not only to restore full power, but also to prevent possible damage to the battery. Ordinary automotive lead-acid storage batteries must be kept at or near full charge to keep the cell plates from becoming "sulfated" (a condition that occurs if the battery is run down and left in a discharged condition for more than a few days). As sulfate builds up, it reduces the battery's ability to hold a charge and supply voltage. Eventually the battery becomes useless and must be replaced.
The charge level depends on the concentration of acid inside the battery. The stronger the concentration of acid in the water, the higher the specific gravity of the solution, and the higher the state of charge.
On batteries with removable caps, state of charge can be checked with a "hydrometer." Some hydrometers have a calibrated float to measure the specific gravity of the acid solution while others simply have a number of colored balls. On the kind with a calibrated float, a hydrometer reading of 1.265 (corrected for temperature) indicates a fully charged battery, 1.230 indicates a 75% charge, 1.200 indicates a 50% charge, 1.170 indicates a 25% charge, and 1.140 or less indicates a discharged battery. On the kind that use floating balls, the number of balls that float tells you the approximate level of charge. All balls floating would indicate a fully charged battery, no balls floating would indicate a dead or fully discharged battery.
Some sealed-top batteries have a built-in hydrometer to indicate charge. The charge indicator only reads one cell, but usually shows the average charge for all battery cells. A green dot means the battery is 75% or more charged and is okay for use or further testing. No dot (a dark indicator) means the battery is low and should be recharged before it is returned to service or tested further. A clear or yellow indicator means the level of electrolyte inside has dropped too low, and the battery should be replaced.
On sealed-top batteries that do not have a built-in charge indicator, the state of charge can be determined by checking the battery's base or open circuit voltage with a digital voltmeter or multimeter. This is done by touching the meter leads to the positive and negative battery terminals while the ignition key is off.
A reading of 12.66 volts indicates a fully charged battery; 12.45 volts is 75% charged, 12.24 volts is 50% charged, and 12.06 volts is 25% charged.
CAUTION: Do not attempt to recharge a battery with low (or frozen) electrolyte! Doing so risks blowing up the battery if the hydrogen gas inside is ignited by a spark.
Your charging system should be capable of recharging the battery if it is not fully discharged. Thirty minutes or so of normal driving should be enough.
If your battery is completely dead or extremely low, it should be recharged with a fast or slow charger. This will reduce the risk of overtaxing and damaging your vehicle's charging system. One or both battery cables should be disconnected from the battery prior to charging it with a charger. This will eliminate any risk of damage to your vehicle's electrical system or its onboard electronics.