Earl:
Having been a fireman on the UP during the steam days, we were taught to always use the try cocks as a check on the engineer's water glass. To me it always seemed effective in that it would properly show consideable reaction in the water glass when cocks were opened. This then could be used to verify the response of the glass reading and to prove the accuracy of the fireman's glass by comparison. But to use try cocks as the sole method of determining boiler water level would be downright scary to me.
Les Clark