The most important thing to understand about testing batteries is that batteries will not show their correct voltage unless they are under a load. If you simply take a multimeter and connect the leads to each battery terminal, you will not get an accurate reading.
Battery testers are designed to place the battery under a load in order to get an accurate reading and are best for normal consumer use; however, multimeters provide a more accurate reading of the voltage potential. If you are needing a high level of accuracy (such as for design or testing), then you should use a multimeter. If you are just wanting a device to let you plug in a battery and know whether or not it's still good, a battery tester is a better choice.
When testing batteries with a multimeter, you must have the meter set to measure DC voltage and place the battery under load by using a resistor in parallel with the test leads of the meter (see illustration at right). If the voltage reads the same on a multimeter with the resistor or without, then the battery is either low or dead and should be replaced.
A battery can be used until it is down to about 30% capacity. After that, it typically will not supply enough voltage to power the product it is being used in. The actual cut-off voltage will vary with the battery type.