The Gibbl-O-Meter should be able to measure the Pressure inside a Boiler, just go with the Scale Mode (Circle with a Dot in it) and measure the Redstone until it reaches 14 (15 would blow up).
Also did you know that at a certain point in the Yellow Range, Boilers emit more Steam to depressurize faster? This CAN lead to some sort of Boiler Overclocking, which most of the time you miiiight want to avoid.