At our vet, we always call to authorize any tests performed after the drop-off unless:
A) It is an emergency situation and the test results are needed stat - we run the test and then call in that case. This is usually only if the cat is failing rapidly and there is no apparent cause. If the owner objects, the vet explains the charge and usually cuts a deal - charges it as a serial instead of a complete panel for example.
B) If the owner is not available, we will SOMETIMES go ahead and run the tests if we are absolutely sure that the tests are vital to the treatment of the animal and the results are needed stat. We once had a chihuahua boarding with us that was (unknown to us) pregnant. We ended up doing an emergency c-section and going to the pet store to buy milk replacement. We couldn't reach the owner, so we had to do everything we could to save both the mother and the puppies. The puppies died, and when the owner returned, he was absolutely livid that we were charging him for these things. He knew his dog was pregnant (had bred her in fact) and so dropped her off to have her puppies with us. He thought that he would get away with paying for just boarding and come back and act suprised when she had a litter of pups! - Just an example of what I mean in this instance...
C) If the owner, upon dropping the animal off, says do ANYTHING you have to do to save my baby don't worry about the cost. In this case, we will run only what is necessary to treat the animal and use our best judgement.
In all cases we contact the owner multiple times giving updates, leaving messages, and doing everything we possibly can both to save the animal and to keep the owner satisfied.
Most veterinarians are pretty willing to negotiate, especially if you can point out that they are in error. However, if they believed that without the tests/treatments that they performed the animal would die - it is hard to punt on the payment because you didn't authorize the procedures.
All in all, if it weren't an emergency situation and you WERE reachable, simply tell them that you did not authorize the treatments and if they wanted to perform the tests they should have contacted you. Most vets in this situation will back down and negotiate. They will probably still want you to cover at least cost on the tests but may not charge you for the exam fee or one nights hospital stay. It really depends on the veterinarian and the condition of your kitty.
Good Luck and I hope this helps!