At minimum there should be some basic testing like for therapy dogs where you would have an in-person exam and receive a certification and tag. I have trained a therapy dog before and any dog that would be used in any service capacity should easily be able to pass the therapy dog test. Any legit service dog should be well behaved enough to ignore distractions, stay with it's handler, and follow basic commands.
That wouldn't eliminate people from faking a need for a service dog, but at least those "fake" service animals would be much better behaved when their owners take them out in public than many of the dogs I see now. And it would give business owners something they could easily ask a guest to show proof if they have someone enter with a questionable dog.
how would a standardized test be developed and who would develop it? Who would administer such a test and what government agency would certify that person? There must be a government licensing board in place to make these decisions, so who would be placed on those boards and who would approve their placement (would they be elected or would the state governor appoint them and if the governor decides then does the legislature need to vote to approve such an appointment?)? Now, once an animal is certified or licensed or whatever designation is necessary...what to do when there is a perceived violation, such as when a service animal or their handler commits some sort of violation of the rules that the board has established? well..there would need to be a review...and who would do that?
Other questions I have because several have mentioned that there "should" be a certification process:
How much does this cost and who pays for it? Does the disabled person incur the cost, after all other licensing situations require the licensee to pay the fees...so is this something that the disabled person is now responsible for?
What are the consequences of an infraction...I assume there will be infractions because if service animals are to regulated that means there must be rules and whenever there are rules, they will be broken. So...are there fines, or some other sort of consequences?
what happens to the animal that loses their license for whatever reason?
Should there be a requirement that the handler carries insurance related to this license or certifications (after all, most other situations where a license is required, the person must maintain insurance)..and who pays for that?
I think everyone who is saying that a service animal must be certified or licensed has not thought about the costs and the oversights involved in such a demand. All of this would essentially eliminate many people from being able to ever qualify their animal. It's already expensive and there are many issues to deal with when getting a service dog in the first place. All your demands for certification makes it that much harder for the disabled person.
As far as the zoo is concerned....they are not exempt from following ADA guidelines. they can ask the appropriate questions and the can limit access to certain areas as this is provided for in the ADA. But they cannot demand things of the disabled person such as a particular harness. But...if the animal is not behaved, if it is a nuisance or causes a problem, the ADA does allow that animal to be removed based on their behavior.