How to know if your pet has weak sight?

If your dog is sniffing around for food vs. seeing it first, then that is a possibility.   But it could just be something your dog is detecting that you can't.    A couple of things to ask yourself about your dog:  Is your dog more easily startled by things that were detected before?    Does your dog seem clumsy when it starts to get dark in the house, especially if something not usually in the way has been placed there?

Hope things work out well.  Your Vet should be able to test your dog to get a definitive answer.