Some people like to monitor volcanoes by constantly monitoring gases that come out of fumaroles. Most active volcanoes have fumaroles where volcanic gases escape to the surface. It is relatively easy to monitor the temperatures of these gases, and an anomalous increase in temperature might be a sign that magma has moved closer to the surface. Monitoring the composition of the gases is more difficult to do, and changes in the composition are way more difficult to interpret. Many times just visual changes to fumarole areas are indications of impending activity.
Many volcanoes have summit lakes through which heat and gases rise to the surface and escape.
Many of these lakes have strange colors due to all the dissolved minerals in them, and many of the colored ones change color, pH, temperature, etc. These too, are signs of change below but are often difficult to interpret. A number of people are studying ways in which to use satellite data to monitor volcanoes. It is possible to obtain thermal images of volcanic areas, and by comparing images on a monthly or bi-weekly basis, increases or decreases in temperatures can be detected. Additionally, some new technologies have allowed for the determination of very accurate topography from satellite data.