You can't tell whether an animal is a service animal just by looking at them. There's no required badging.

(Though I have modest sympathy for people trying to skip the carrier step--the U.S. special services desks I've dealt with to pay for and get my dog's carrier tagged have been actively moronic on the subject, even though they must see a few dozen dogs a day. [I said "modest," I do understand why an untrained animal needs to be in an underseat carrier!])