Let's just say.... if the photographer is named Don... and he asks for your ID.... you should run for the door!![]()
OK... that was seriously funny.

Mickey88 is absolutely correct. A well-worded contract will allow the pro photog to make that determination on the spot.
There are some photogs that go as far as to have it written in their contract that no guest may take any photos of their posing, their set ups, or while the pro is shooting. Some just say they don't want any other pro's or people masquerading as pro's trying to work behind the scenes. It is really up to the individual photographer and you'll notice that a lot of contract language is a direct result of photographers who have been burned in the past by different people/guests/couples.
So it really does come down to the individual photographer and their own contract.
Personally... if a guest walked into a wedding with their D700 and a 24-70 f2.8 and was just taking pictures that were obviously just personal - I wouldn't care. However, I would take issue when that person started taking pictures of my posing or interaction with the couple, started moving around during the ceremony, and started acting like they were building a portfolio or were just getting in my way.
And it depends on the individual temperament of the photographer as to how they would handle the situation. Some would just get up and walk, some would mitigate the situation somehow, some would talk to the bride/groom, it's an individual choice.
I did a blog post where a couple of my girlfriends submitted their pictures of those overzealous guests. They obviously didn't walk from the wedding... but not everyone is so nice.
http://www.whimsicalphotodesign.com/gh/page/2

but I dont want to get 'ripped off' either. 

Most people don't really care what your degree is in, as long as you provide awesome pictures and you give them what they want.