How to predict a volcanic eruption

Before you do anything you need to investigate the history of the volcano. You can tell alot about a volcano from its eruption patterns.

If the volcano is smoking, examine the gas being discharged, most volcanoes erupt after a change in temperature or gas. This isn't true for all volcanoes though and weather conditons can affect volume and temperature too.

Magma (molten rock) moving towards the surface can predict eruption, this can be seen as bubbles or tested with a devise called a magnetometer. Aerial infrared photography can help you detect the temperature over a period of time, along with recordings of earth tremors, eruptions are generally preceeded by a long series of small earth tremors. While none of these factors by itself can be used to predict an eruption, the presense of many can be a sign something may be about to happen.