A battery can be tested using a multimeter or a dedicated battery tester.
Simple battery testers are often used for AA and AAA batteries and indicate whether the battery still contains sufficient charge. Basic models typically use an LED indicator, while more advanced testers measure the actual battery voltage.
A multimeter can also be used to check battery voltage. For example, if a 1.5 V battery measures only 1.2 V, this may indicate that the battery capacity has significantly decreased.