Dogs usually adapt to vision loss so well, it's hard to pick up on. The first signs are usually hesitation going in/out (light changes) or going up/down stairs. One test you can do at home is to take some cotton balls and (when you have his attention) gently toss them at his head and see if he moves/flinches/tracks them. 5 is when something like Progressive Retinal Atrophy will start showing up, so I'd at least do some home vision checks. PRA isn't common in greyhounds, but there are a couple prolific sires that are carriers, so it's not an impossibility.
Exactly. The first sign that Phene was losing his sight was he stopped going upstairs. He can still see, but we think he only sees movement. A test like Heather described wouldn't work with him because he still tracks movement. The vet determined that something was wring by looking into his eyes with a bright light. Does Jack have a vet appointment in the near future?