If you shine a light into your guinea pigs eye and it shines a white color instead of red, like a normal guinea pig eye, that is a big indicator that your guinea pig could be blind. Also you can watch them explore an unfamiliar environment and if they are running into things or seem lost, it could be a sign of blindness.If you are really unsure then ask a vet, that's how I found out my guinea pig Gus was blind.
Try not to change around where the food dish, water bottle, hay rack, etc... is because they might not be able to find them and won't eat or drink as much as they should.
Guinea pigs who are blind may startle more easily then pigs who can see. So this can make taming very hard. I have found that if I talk to Gus, he can tell I'm coming and he tends to be calmer when I approach.
Gus tends to always be relying on the other guinea pigs to lead him around, and he's a lot more confident when he can feel them with him. When he gets lost he can hear the other pigs and will go running across the cage to meet back up with them.
Microphthalmia is a congenital condition that a guinea pig can be born with, where the eye is either very small or non-existent, making the guinea pig blind.
I have to do certain things for Gus to make sure he's eating as much as the other guinea pigs, and that they don't snag his vegetables from him at veggie time. I also try to get hidy houses with big openings because I've found that sometimes he will go into a hidy hut, then not know how to get back out. But other than that he's just like the rest of the guinea pigs.