Service Dogs

I feel like people are very rude and get all defensive when you tell them that dogs are not allowed in the building. The first thing I get is "you know that it's against the law for you to deny entry to a service dog," well how I'm I supposed to know it's a service dog if it doesn't have a vest or special leash. I understand that it is not required for them to wear it so people should not get all defensive about it and just say they are a service dog and be more nice about it.