We're trained professionals

According to the NHTSA Manual entitled DWI Detection and Standardized Field Sobriety Testing, when conducting the horizontal gaze nystagmus (HGN) test, the officer is to check the suspected drunk driver's eyes for pupil size, resting nystagmus (involuntary jerking) and tracking ability. The manual states that if "the eyes do not track together, or if the pupils are noticeably unequal in size, the chance of medical disorders or injuries causing the nystagmus is present."

Presumably this would also include drivers who are blind in one eye.

Not, however, according to Officer Chris Murray of the Pasadena (Texas) Police Department. On December 14, 2008, Officer Murray, while offduty, stopped a driver he suspected of driving while intoxicated. Per Officer Murray:

I performed the Horizontal Gaze Nystagmus test on the suspect, and during the pre-test check, I noticed that the suspect's left eye appeared to have some sort of deformity. The pupil was not readily visible, the brown part of the eye was cloudy. I asked the suspect about the problem, and he told me he was blind in one eye. The suspect's eyes however, tracked normally. The suspect had a lack of smooth pursuit in both eyes, a distinct sustained nystagmus at maximum deviation, and an onset of nystagmus prior to forty-five degrees. The six clues (three for each eye) were a strong indicator of intoxication. (emphasis mine).

It's common place to see every officer claim to have observed all six clues for every driver they test, but it's a bit out there to find an officer who claims to have observed these clues in an eye out of which the driver can't see.