I'm visualizing an airline-only response (as opposed to State licensing) that involves on-site behavior testing and adds inconvenience to the mix. Someone, perhaps even a trained volunteer, takes your service animal to the pet area and runs it through a set of behavior tests - while you wait. (Maybe not the pet area but someplace land-side before you go through security.) An assessment protocol would need to be designed for various animals - dogs, cats, ferrets, peacocks, miniature horses etc. Your animal only needs to complete the assessment once, but all animals are required to be chipped so that you can't substitute a different animal. You could do the assessment on a different day from your flight (but, like Global Entry, it might take awhile to get an appointment.) There would be a fee for the assessment (after all, responsible animal ownership is expensive.) Volunteer evaluators might be possible because lots of people like to work with animals; they could be recruited from local animal shelters. However, it would take some expensive animal behavior consultants to design the tests and train the volunteers. Gradually a system develops that's trustworthy (as opposed to mail-order "Emotional Service Animal" certifications. I'm even wondering if an animal's fitness score could be stored on its chip and used in different contexts. Wondering if ADA laws could be modified to address these issues.