Cover up the eye that you think is good, and see if it stumbles around blindly. If not, cover the other eye.
If he's blind in one eye, he will be swaying and turning his head a lo more than usual. Also, if one eyes looks hurt or swollen, but he's not behaving unusually, he most likely still has some vision in that eye. Just maybe not full vision ...
Pretend to punch him in that eye and see if he flinches . Or you could take him to his vet. I'm not sure what their eye test is like since most can't read (Yamster excluded).
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