Gender: Unspecified Location: IN YOUR MIND...MUWHAHAHAHAHAHAHA
The Avengers and the FF are very public teams and people know they are not mutants because they told people they weren't. Most superhumans are on teams like this, while most mutants are trying to live like ordinary people until something with their powers happen. For the most part people don't know unless the superhuman is on a team like the two mentioned. Skin from Generation X was saving a guy who was on a boat attacked by Fever Pitch, they guy asked if Skin was a mutant because he didn't want a mutant to save him, Skin told the guy that he got his powers from a radioactive elephant bite, so the guy was cool with it.
I've always wondered about this myself. How does the average guy on the street know the difference? If I was a mutant I'd just make up some dumb cover story, go join the Avengers or something, and be be a real super-hero, not some hated loser. In the MU there are lots of options:
Cyclops - hit in the eyes by a radioactive cannister while saving an old man from being run over
Nightcrawler - alien
Wolverine - experiment with using animal DNA to create a super-soldier or bitten by an escaped rabid radioactive experimental lab animal
Storm - hit by lightning and gained weather powers, or magic user.
Iceman - caught in an industrial coolant leak, or technology user like Jack Frost, Blizzard etc.
Havok - changed by cosmic ray bombardment in experimental space flight
Colossus - techno-virus
Marvel Girl - gained powers during strange mind experiments at college
Kitty Pryde - technology user
and so on...
The various in-jokes in Marvel over the years like "maybe I was bitten by a radioactive something-or-other" always made me wonder - do people know that's how Spidey got his powers? Before Civil War I mean.
As for the mutants, the only thing I can think of is that people like the Government (using Sentinels & other mutant detecting tech) and anti-mutant groups with good resources "out" these guys publicly. Hell, maybe Professor X secretly leaks the info so that they all come crying to him and he has his little empire to run and can be the "saviour" figure.
And when MU citizens can probably log on the internet and find out who is a mutant there isn't much point trying to hide it.