How can you tell when a dog is starting to loose there eye sight?

Obviously when they start bumping into things but I mean looking at there eyes is there any certain colours which will show that they are loosing there eye sight?

I think my dog might be losing her eye sight slightly, she has brown eyes but the black bit in the middle has spots of blue But im not sure if this is a sign or not?

Any info please I dont want my baby to loose her eye sight.