Oh by the way, I called 5 diffrent vets in my city today, they all said about 98% of the time you can see a scar from a past surgery. They also said in the rare chance that you didn't, there's no way they should be charging me full price for the surgery ESPECIALLY AFTER THEY TOLD ME THEY COULD TELL 100% OR NOT IF THE CAT HAD ALLREADY BEEN SPAYED!
		
		
	 
Well of course, they are competitors, what did you expect them to say?  If you call 
any business and tell them about a negative experience you had at one of their competitors, they're going to say "Oh man, that is just wrong.  JUST WRONG I SAY!  We will never treat you that way here" or something to that effect.
My mother finds homes for unwanted dogs and cats rescued from various sources.  She fosters over 100 dogs and about 30 cats every year (she doesn't like cats as well), most of which have unknown spay and vaccination histories.  Every animal must be sterilized before adoption unless they are too young.  Because her eyesight isn't all that great, she has me shave the animals (if necessary) and look for incision scars on the females.  I suppose the fact that I was a surgical assistant for several years has something to do with it, too.
In the last two years, we have had several animals with no discernable scar that were already spayed.  A few of which we were able to find spay information on before taking them to the Vet.  Three of them went under the knife and my mother was charged the full price each time.  We work with three different Vets in the area.  There is no Vet in this area who would not charge full price for this, because 85~90% of the total cost is putting the animal under anesthesia, prepping and draping, opening the animal's pelvic cavity, closing it again, and post-surgical care.
While I agree there is no cause for the Vet to get rude about it and they seem to have atrocious customer service skills, you really don't have a case since your decision to have the procedure would not have been different had you been told 98% instead of 100%.
Pay the bill and use another Vet for all your future needs.