Whilst I agree with what has been said about grey hairs being the main interest in steam, I think there is room for some basic info and tips.
Here is my input on sight glasses:
Why have them?
When operating a boiler, one of the most basic safety requirements is to have water in it, otherwise 1) no steam, 2) potentially melting boiler! So, we have a sight glass to see the water level.
In principal, if you cant see the water level in the glass, do not light the burner - simples! Or is it? If you have overfilled the boiler without realising it and you now cant see the level, how can you be absolutely sure there is water in it? I know you know you have just filled it, but are you REALLY sure?
This was drummed into me as an apprentice so deeply I am almost paranoid about it - which has helped a number of times as I operate full size high pressure boilers for a living.
So, how to find if the glass is full or empty? easy, using the principals of refraction, get a straight piece of wire, or thin pencil or similar, hold it at an angle behind the glass. If its empty the straight object will look straight, if the glass is full, you will see the part behind the glass 'bent' at a different angle to the part not behind the glass.
Remember - NO WATER = NO FIRE