PPS-seeing eye dogs have special harnesses and a giant sign on them that says "Service Animal"
You can actually buy those vests on amazon. They have velcro labels for pretty much every type of service animal, including "emotional support animal," K9 Unit, etc. My brother got me one for my english bulldog (with just the "service dog" label). I've only had the balls to use it once. I took him to a hardware store. My dog started rearranging the merchandise. I tried to pass it off like he was in training but I really couldn't keep a straight face.