How you handle it depends a lot on your own history and your comfort with training. There are a number of certifications you can get for dog training, but in the US, dog training is an entirely unregulated industry where anyone can say they are a trainer without any background or skills at all. I never represent myself as a trainer to clients because to me, a real trainer is someone who has gone the extra mile to learn from professionals and has sought out and understands the current research on canine cognition and behavior. They likely have a certification, or extensive experience in the field. I am someone who has put in a lot of effort on my own time reading the research, studying videos and blogs, but I don't have the amount of practical experience that I would consider proficient to consider someone a trainer (though I'd love to someday!). All that said, I discuss with my clients if there is anything they are working on with their dog and I'm happy to keep up with their current routines (provided they are using humane methods). I am also happy to work on a variety of basic issues, like not charging the door, not barking at the neighbors, leash manners, etc, but I mostly see that as a way of making the stay safe and manageable rather than as providing training. These are just skills I want dogs in my care to have, and I'd encourage desirable behavior regardless, so I put in the effort to show guests what's expected of them. I care a lot less about classic "obedience" training. Beyond sit, a basic stay/wait, and good recall, I don't see much of a point to it, especially as a sitter. Good manners without direction is far more important to me than a dog I can boss around.