To What Extent Can Volcanoes Be Successfully Predicted?

•   Seismic activity (small earthquakes and tremors) always occurs when a volcano awakens and prepares to erupt. Some volcanoes normally have continuing low-level seismic activity, but an increase can signify an eruption. The types of earthquakes that occur and where they start and end are also key signs. The seismograph uses a pendulum to record movement of the ground below it. The squiggly lines recorded on paper by a seismograph are called a seismogram.
•   Since magma gives off electric currents, electric meters are used to spot rising magma levels by measuring its electric current. Gravimeters can also detect flowing magma.
•   Scientists also take temperatures by using a Landsat satellite. The satellite uses infared sensors to detect temperature changes in a volcano.
•   As magma nears the surface and its pressure decreases, gases escape. Sulfur dioxide is one of the main volcanic gases, and increasing amounts of it usually mean the arrival of more magma near the surface. Aircrafts monitor the amount of gas released from the ground. An increase in sulfur dioxide and other gases usually means that there could be volcanic eruptions.
•   As magma moves up into a volcano the volcano swells, and distances between points on the volcano (think of dots on a balloon) as well as the elevation at specific points increases. Such a change is a common occurrence prior to an eruption. The tiltmeter is a sensor that uses a laser beam to find the rising or lowering of magma levels by measuring changes in ground elevation.
•   Before an eruption, magma moves into the area beneath the volcano and collects in a magma chamber. Magma gathering in a chamber causes slight swelling of the volcano's slopes. Swelling of the volcano signals that magma has accumulated near the surface. Scientists monitoring an active volcano will often measure track changes in the rate of swelling. Radar satellites that continually orbit the earth can detect as little as a one-millimetre increase in swelling on the flanks of a volcano. An increased rate of swelling--especially if accompanied by an increase in sulfur dioxide emissions and harmonic tremors--is almost a sure sign of an impending erruption.
•   Hydraulic measurements are increasingly used to monitor changes in a volcanoes. Increased gas pressure will make water levels rise and suddenly drop right before an eruption.
•   Volcanic deposit that may be placed on a river bank can easily be eroded which will dramatically widen or deepen the river channel. Therefore, monitoring of the river channels width and depth can be used to assess the likelihood of a future volcanic eruption.
•   Pre-eruption sediment may be picked up by a river channel surrounding the volcano that shows that the actual eruption may be imminent. Most sediments are transported from volcanically disturbed watersheds during periods of heavy rainfall.

Sources:
http://en.wikipedia.org/wiki/Prediction_of_volcanic_activity
http://www.yourdiscovery.com/science/eruption/predicting/index.shtml
http://www.learner.org/interactives/volcanoes/morfore.html
http://library.thinkquest.org/J003007/Disasters2/volcano/predicting/predict.htm