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You need to look at the pressure gauge (usually) on the boiler. You should have at least 10 to 12 psi when the system is cold.
It's not unusual for the gauge on the boiler to fail. You need to get a test gauge like this onehttp://www.homedepot.com/p/Watts-3-4-in-Plastic-Water-Pressure-Test-Gauge-DP-IWTG/100175467
But one with a lower scale would be easier to read for this application. Screw it onto any hose drain/faucet on the system and open the drain to check the actual pressure. Anything over 30 psi and the relief valve should discharge. Normal operating pressure range (from a cold to a hot boiler) is 10 to 25 psi.