You should always let the hotel know about your dog before checking in. This way, they can ensure that you are in a pet-friendly room, plus inform you about anything else you may need to know. If booking online, there's typically a "bringing a pet" filter to use. Once you check-in, you will likely have to fill out a waiver agreeing to the hotel's pet policy. Sneaking your dog in a pet-friendly hotel could cost you a lot more in the long run.