How can volcanic eruptions be predicted?
Past data and trends and readings.
Eruptions can be predicted using the time usually between eruptions, and then seeing how long ago the last eruption was then working out, using that data, if it is imminent, then an eruption is expected. (Past data and trends)
Also, if there is an increase in volcanic activity etc, that is often a good, but short-notice, warning of a volcanic eruption. (Readings)