How could you tell?
She should, voluntarily, submit to standardized neuro-psychiatric, IQ testing. A significant deviation from the mean is enough to conclude whether or not she has an impairment. Her constituents should demand it. Otherwise, as I have indicated, constituents should demand that she debate her opponent in a high profile venue. Again, this should be voluntary.
If AZ residents want to re-elect someone that is severely intellectually impaired, out of a feeling of irrational sentimentality, that is unfortunately their prerogative.