How can they tell who's a Muggle?/Masons

[Jim Ferer <jferer@yahoo.com>]

No: HPFGUIDX 52366

Greicy:"I've come to a halt. How can Wizards, particularly the DEs,
tell who is a Muggle? It can't be because they wear Muggle clothing
because the Weasley kids wear Muggle clothes. The only thing that can
give a Muggle away is if they speak to someone and show them their
lack of knowledge about the WW. Unless you're Hermione."

If you're speaking specifically of the QWC scenes, then it's easy: the
Roberts's were the camp owners that everyone paid and passed by to get
in. Everybody knows they're Muggles.

For everybody else, my guess is there's several answers:

We've had a lot of discussions about the population of the wizard
world, but it's not a large one, so there's a tendency for everyone to
"know" everybody else, or at least a relative.  There's also a lot of
non-verbal signals, too, that people spot.  Mr. Weasley was dressing
up in Muggle clothes for the QWC, for example, and Harry wasn't
impressed with Mr. Weasley's "disguise." 

People who wear Muggle clothes every day look like they do. This kind
of cultural crossover might well make up part of the tension between
Muggle-borns and "purebloods."

It even happens in the Muggle world: I bet if you and I walked down a
street in many foreign countries, even where one or both of us was the
dominant ethnicity, we'd stick out like sore thumbs.

Finwitch:"I think that the Masons are magical, and that it was truly
one of them doing the Hoovering Charm, thus scaring Dobby away.
Dursleys don't know, the letter was sent to 'the wizard who did magic
at Privet Drive 4' - logic told the writer it was Harry, but magic
told the owl it was Mason..."

Well, that's original, anyway. What led you to that belief?  I think I
prefer the simpler explanation that what the scene looked like is what
it was.

Jim F.