If a volcano has not erupted in a long time how do scientists know that it is still ‘live’?

A volcano is considered active (i.e., “live”) when signs of activity can still be detected. These signs can be occasional volcanic seismic activity, gas emitted on an around the volcano, ground deformation. Apart from the occurrence of gas vents, most of these signs are generally only detectable with specialized monitoring equipment (e.g., seismometers, GPS).