Dan, you are certainly on the right track, and I will see if I can shed a bit more light on the subject, just for clarification, if nothing else. I've been a full size engineer in a stationary plant for about 15 years.
ASME code in the USA requires two ways to check the water level in all boilers of more than 10 square feet of heating surface. The number cited here is from my memory and may not be entirely correct, but I did not take the time to look it up, as in most cases it doesn't really matter for the small machines we build. Now, (as mentioned before) in boilers of less than 250 PSI, try cocks can be used as the second method instead of fitting a second sight glass to the unit. In higher pressure units, two sight glasses are required. New full size boilers of less than 250 PSI are still fitted with try cocks if a second water glass is not specified by the customer.
The use of try cocks is much simpler than you might expect, and you certainly do not just wheel one open. Placement of the cocks are as follows:
The top cock is mounted to the shell at such height that it should always have steam issuing from it when opened.
The middle cock is mounted at the desired water level, and should issue a mix of steam and water when opened.
The bottom cock is mounted at the lowest permissible water level, typically two inches or more above the highest point of the crown sheet, it should always issue water when opened.
In practice, one opens the bottom cock first, and just barely cracks it open. Water drops will issue from the cock if the water level is high enough. Once the operating engineer has satisfied himself that he has sufficient water in the boiler for continued operation, he can try the middle cock, which when cracked will issue water droplets but will also give some "sizzle" as steam escapes. Ideally, you should get very wet steam, or a better description would be a mixture of steam and water. The top cock is tried last, to make sure that the water level is not too high, and when cracked it will give steam immediately if the water level is correct.
The assumption that water escaping from any of the try cocks will instantly flash into steam is not entirely correct, but reading the cocks does take a little bit of practice and in full size practice they need to be tried regularly simply to be sure that they are not sealed shut by mineral scale or mud. They are not used at pressures above 250 PSI because at elevated pressures it does become more difficult, and eventually impossible to tell what is issuing from the cocks, as water will flash into steam at those pressures, even if only cracking the cocks open.
Do I have a personal preference in the cocks vs. no cocks question? No, not really. I will admit that having two sight glasses mounted to two separately plumbed water columns is preferred, but trying the cocks on a regular basis keeps me in touch with "the old days" too.
to have had experience with try cocks and IMHO they may work at lower pressures where the difference between steam and water is not all that great but at higher pressures I doubt their efficacy, ( Perhaps that's why they're not fitted any more).
