[header] How to diagnose canine corneal ulcers [title] Be mindful of your dog rubbing her eye. [step] Ulcers can be difficult to see with the naked eye. Since corneal ulcers are painful, your first clue that there is a problem is that the dog's eye will bother her because it hurts. 
A typical sign of a painful eye is rubbing at the eye. [substeps] Your dog might do this by rubbing the affected side of the face along the ground or rubbing at the eye with a paw.