They check to see if the top of the volcano is plugged with cooled lava. An active volcano is one that has erupted recently or volcanologists see the possibility of an eruption soon. Active volcanoes are watched very closely so people and animals in the area can be moved to a safe place. A dormant or sleeping volcano is a volcano that has been quiet for a long time, but still has signs it may erupt again. A volcano becomes dormant when the vent is blocked by hardened lava, called a plug, or if the magma seeps back under the earth's crust.
Volcanoes can be dormant for hundreds of years.
Then suddenly a volcano will erupt again. The eruption is usually very violent. The plug of a dormant volcano stops the magma from rising. Then pressure under the plug builds up, so the plug gives away and a large eruption is caused. The volcano is then classified again as an active volcano.