Monitoring

Electro-magnetic variations have been observed after earthquakes, but despite decades of work, there is no convincing evidence of electro-magnetic precursors to earthquakes.
There are a few historic examples of simultaneous eruptions from volcanoes or vents located within about 10 km of each other, but it's very difficult to determine whether one eruption might have caused the other.
Geothermal energy (heat energy from the Earth's interior), is used to generate electricity in a variety of places throughout the world.
Scientists agree that drilling into a volcano would be of questionable usefulness. In addition to the enormous expense and technological difficulties in drilling through hot, mushy rock, drilling is unlikely to have much effect.
If another catastrophic caldera-forming Yellowstone eruption were to occur, it would probably alter global weather patterns and have enormous impacts on human activity (especially agricultural production) for many years.
Instruments to measure sulfur dioxide and carbon dioxide can be mounted in aircraft to determine the quantity of gas being emitted on a daily basis. Such instruments can also be used in a ground-based mode.