Would a dentist notice gum disease just by looking at them?

Diagnosis is simple. Diagnosis is based on your description of symptoms and an exam of your mouth. Then X-rays are needed to check for bone loss and to support the diagnosis.
Gum disease. Depending upon the severity, the length of time you have had periodontal (gum) problems ; your body's recuperative powers, merely by 'looking at them' may or may not indicate its presence. I have seen patients with severe bone loss whose gums look normal, while others with severe redness ; swelling that respond easily ; quickly to treatment. X-rays ; probing are also essential for diagnosis.
Possibly... But visualization may only show a presence, not the severity. A clinical exam, coupled with a full mouth of radiographs and pocket depths will give a clearer picture as to both the amount of damage and treatment necessary to correct the soft and hard tissue defects.