I was told by my vet that as the dogs age, there are small changes to head and eye shape that can impact the ducts and cause more tearing. I have the same issue with my old guy (not yellow so it is less of a stain than wet streaks) who is 10, but I still CERF him and his eyes have continued to check out fine.
To answer your second question - vets check for glaucoma doing a test called tonometry. It means that a small handheld instrument with weights attached is actually touched onto the eyeball to measure the pressure inside the eye. The newer handhelds are digital machines and they are much more accurate.
It isn't painful and is done with the pet awake and not sedated .
I will pass this on as I heard it years ago from a woman that had a Maltese coming in for boarding where I used to work. She brought jugs of distilled water and requested we give that water to her rather than the tap water at the kennel. "Why" we asked. She replied, "It clears up and prevents the stains under her eyes". She was a show dog at one time and that was what the handler told her to do to keep her eyes pretty. I will say this dog did not have any tear stains which are so prevalent on all little dogs with long coats and big eyes. I would guess there are no minerals like iron in it. It makes perfect sense to me.
