Some points move upward and any two points will move farther apart. By using very accurate leveling and distance-measuring techniques, these surface changes can be measured. Usually the changes are a few mm over a distance of a few hundred meters, but sometimes they are dramatic. For example prior to many eruptions at Kilauea, the summit bulges 1-2 meters upward. 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.