How do I know if my dog is losing his sight?

You'd be shocked that your dog is losing or has lost its vision. Unless it happened really quickly and it was both eyes at the same time, your dog can adjust to so many things. I used to work at a place where they had a blind cat and nobody knew the cat was blind. The cat had adjusted and knew where everything was. It could sense other things around it. So if your dog suddenly loses vision, it may bump into things. If it didn't happen suddenly, or it still has a good eye, it's still going to navigate well. So if something seems weird or off, if they seem to shy away or get startled coming around the corner, those could be some signs.

Different diseases can cause blindness. High blood pressure, kidney disease, thyroid disease, and such things can affect the eyes in ways that can create other symptoms.

I had a blind Cocker spaniel and if you didn't know him, you wouldn't know he was blind unless you moved the furniture a bit. And even then, they get a sense as they're moving, they can feel its presence on a different level. They can sense things. I think we've spent a lot of time moaning and groaning about why we can't see as well, and they just adjust. They have to adjust to survive.