According to the University of Utah, the new reservoir was discovered using seismic imaging, which is something like a CT scan, but with earthquake waves instead of X-rays. Basically, they were able to detect magma because earthquake waves travel faster through regular rock than through molten rock. The Utah-based team also had access to more advanced seismometers that produced higher quality images.
All this scientific information does not mean the supervolcano is any closer to erupting, however. The amount of magma has not increased; it's just the first time anyone's seen it. In fact, researchers say that there is a one in 700,000 chance that the volcano will erupt anytime soon.
But all it really takes is one time for everything to go boom. Just ask the former residents of Pompeii. Which, you can't. Because they all died.