What exactly does a doctor look at to determine glaucoma"?

Eye pressure. Glaucoma screening involves checking the eye pressure, looking at the optic nerve and checking the visual fields. Normal intaoccular pressure is between 10 - 21. A pressure greater than 21 needs further evaluation including dilated eye exam to check for damage to the optic nerve and visual field testing to check for peripheral vision loss. Uncontrolled glaucoma can lead to blindness.
Pressure and more. The hallmark of glaucoma is internal eye pressure greater than it can withstand - so your ophthalmologist will check that pressure. He will also measure corneal thickness, structure of the nerve exit site at the back of the eye (the disc), and measure the visual field. All of these are clues to glaucoma; your ophthalmologist can put this all together for you.