Home heating steam pressure (a couple of psi maximum) would normally be measured from piping open to the boiler tank above the water line, with perhaps a (rarely installed) low-pressure dial-gauge, or a device (even a small U-tube manometer) to be connected during professional maintenance.
Here asking about an anecdotal "quick-and-dirty" rough-estimate procedure: Cool the boiler, attach a clear tube (say 1/2" ID) to the drain outlet, stabilize it vertically, open the drain-valve, mark the water level in the tube, re-start the boiler, and measure the difference in the height of the water column in the tube to roughly "read" the steam pressure (pushing down on the water in the tank) as per a change in the tube of "~27 inches water height = 1 psi".
Assuming this proposed mechanism can even work, for such a rough idea of pressure suspect things like water temperature density differences, surface tension, or that the boiler may not be a sealed vessel would not be of concern, but am wondering not only about the distance-to-pressure scale cited, but about the effect of surface area of the tank water vs the water in the open-end tube. Unless the >1000 times difference in area cancels out some factor, rather than magnifies it.
Thanks. A more complete representation of a steam system can be found here (a plumbing website).