It is often a race to see if Penta will erupt or if Sawmill will erupt. An eruption of either dooms the other. There are two vents on the boardwalk side at the base of Penta's cone. It is possible to watch the water level rise in these two vents. Before an eruption, the water level will eventually rise until it forms a shallow pool that almost reaches the boardwalk and starts overflowing towards Sawmill.
Convection currents form over the vents and bubbles start to rise.
It is possible for the water level to drop at this point only to rebound a few minutes later or to just drop and disappear. It is when the water level is high, there is runoff and there is bubbling over the two vents that the eruption can occur. Unfortunately, even at this late stage, Sawmill can still steal the glory. While the water is rising at Penta, it is also rising at Sawmill. If Sawmill erupts first, Penta will begin to drain and not erupt.