Above are some pictures of boiler gauges, you would think that something as important as your pressure and temperature gauge would be right in front, and easy to read, In some cases it is, in others you need a mirror, and need to bend like a pretzel to read it. So you will need to locate your gauge, one part of the gauge reads the pressure the other part reads the temperature. either one of them can become stuck. First the pressure. it should read 12 psi to 20 psi. 
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I just removed one the other day that is still reading 12 psi (stuck at 12 psi) the boiler really had no pressure at all. But how did I figure that out? I needed to test the boiler relief valve, to be sure it was not stuck (this is very very important )  When I pulled the tap, just a little water came out, this could have been because the relief valve was clogged, so I had to investigate a little further. I turned the power to the boiler off and hooked a hose to the boiler drain on the bottom of the boiler and opened the drain, water came out but with no pressure, and the fill valve did not let water back into the boiler. so now I decide to drain the whole system, (because I know I have to make repairs) I remove an auto vent on the top of the boiler to let air in to the system so it will drain, the gauge is still reading 12 psi, so I had to replace the gauge and the fill valve and the expansion tank. to get this boiler running correctly.

Having pressure in your boiler is very important, Not having pressure can be very dangerous.

 How to test your gauge.

First see what the gauge is reading, write it down, (both temp. and Pressure) Then turn the power to the boiler off, then turn the water off. then hook up a hose to the boiler. and let just enough water out to make the gauge move down to zero. if the gauge does not move it needs to be replaced. if it moves. stop draining water and turn the Water back on, If the pressure goes back to the  12 psi to 20 psi, the pressure part of the gauge is fine. Now for the temperature. leave the boiler off (if it is not heating season) for a few hours to let it cool. (it may not go below 100% f this is normal then turn it on and wait till it shuts off by its self, then check the temperature it should be below 200% f . If it gets higher you may have an Aquastate problem Or your aquastate is set too high. If it does not move at all it needs to be replaced.

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