How do scientists predict when a volcano will erupt?

Question

I am doing this for a lab that counts for a large portion of my grade, so please give me serious answers.

Answer 1

http://volcanoes.usgs.gov/About/What/Monitor/monitor.html
This USGS site will get you to more information about volcano monitoring techniques: remote sensing, gas measurements, ground deformation measurements, seismicity measurements, geophysical measurements, hydrology monitoring.
"Volcano monitoring methods are designed to detect and measure changes in the state of a volcano caused by magma movement beneath the volcano. Rising magma typically will (1) trigger swarms of earthquakes and other types of seismic events; (2) cause swelling or subsidence of a volcano's summit or flanks; and (3) lead to the release of volcanic gases from the ground and vents. By monitoring these phenomena, scientists are sometimes able to anticipate an eruption days to weeks ahead of time and to detect remotely the occurrence of certain volcanic events like explosive eruptions and lahars ."--no copyright because government site.

Answer 2

Magic 8-ball.