This is a question I get asked by nearly every group that visits the mosque. This is a short'n'sweet answer that so far has worked like a charm.(Which of course is a turn of phrase I just realised that is pure shirk and in fact charms don't work! So it works anyway, unlike a charm, but like something that does work..like a Toyota maybe..anyway) it goes something like this:
Moi: "Have you ever seen a nun?"
Questioner: "Yes, sure!"
Moi: "How did you know it was a nun?"
At this stage please watch expression on questioner's face as dawn breaks on it / light bulb goes on.
Questioner:"Because of the way she was dressed."
You can take it further....
Moi: "And don't most people treat them with a certain respect because the modest dress shows that they are religious people.
This is exactly what Allah tell us in the Quran, that the believing women should cover themselves so that people will know that she is a believing woman and she will go unmolested."
read this interesting article: http://www.themodernreligion.com/women/w_hijab_chinesetrial.html