Elemental sulfur (brimstone) doesn't actually smell like rotten eggs though. You're confusing it with hydrogen sulfide.
Sulfur apparently smells similar to a match (admittedly a much more menacing aroma), and I don't think people would be familiar with it without experience in what it is, considering this is a quasi-medieval universe we're talking about. So you might need alchemical knowledge to properly identify it, since a common individual would never have smelled such a thing in all likelihood, while somebody who's worked with brimstone before would.
Wikipedia also tells me that brimstone might refer to burning sulfur, which would produce the suffocating smell of sulfur dioxide. Also probably a match-like smell, but more unpleasant (having accidentally inhaled match fumes before, I speak from limited experience). Brimstone is associated with abyssal, infernal and daemonic things largely because it's associated with Hell - and this association comes from observations of volcanic activity, where sulfur dioxide would be found emanating from the earth. People living near volcanoes in the fantasy world in question would know it as a "get the fuck away or you'll die" signal from collective experience, if the suffocating aroma wasn't enough.
So, list of people who know what brimstone smells like:
1) alchemists - average Craft (Alchemy),
2) people who are experienced with volcanoes - difficult Knowledge (Geography) or a very particular background,
3) people who have been near a fiend in their life - having bothered to smell a fiend before or difficult Knowledge (Planes).
Anywho, since it is a "faint" aroma, being unspeakably flatulent would probably cover it up anyway (that is, unless it is supernatural in origin and deliberately unmaskable).
I think you could, however, work the misguided idea that demons ought to smell like rotten eggs into the world's folklore, and that actual yugoloths and other B.O.-challenged demons would see no need to correct this idea. Folklore should very often be wrong, you know, since it is in essence an amplified rumor.