How (and if) to talk about a dog's heath with owners?

I've run into a few situations where I wasn't sure if it was my place to say something to an owner about their dog's health. A few weeks ago I had a dog boarding that was very healthy and well taken care of, but his ears were really a mess. I didn't see mites, but it looked like they really needed to be cleaned. I ended up not saying anything to the owner because I felt like it ultimately wasn't my place to tell her how to take care of her dog, and I didn't want to seem condescending or imply that she doesn't know how to care for him. I still feel conflicted about it because the well-being of the dog is the most important thing to me. Today I picked up a boarder who has a growth on his eye, and it really looks like it could be cancerous to me. I don't have any credentials in the medical field, but I've had and worked professionally with dogs for a long time and this is concerning to me. The obvious answer seems to be to alert the owner, but I still worry that it's overstepping a boundary somehow. What if I'm wrong and it's nothing? Would it come across as rude? Any opinions/advice would be greatly appreciated!!