They do have to look just to check that what's supposed to be there, is there. Just because they were quick though doesn't mean it's a boy. They look at these scans all day long so they get pretty adept at it.
I remember with DS as soon as she started the scan the sonographer said "would you like to know the sex" and we said yes, she ran the transducer down there and looked for about 2 seconds before telling us he was a boy.
Even if you watch the dvd it's very unlikely you will identify it especially if she only had a quick look. Even when it's glaringly obvious most people need it pointed out before they can make out what they're looking at...then it's kind of "Oh Yeah!! I can see it now!!".