Are You Awake, Mr. President?

The vice president of the United States, Dick Cheney, will be briefly in power for a second time according to this report from MSNBC. The reason for his assumption of power on Saturday is the president will be under the effects of anesthesia while a colonoscopy is performed.

Apparently, a previous operation was performed in 2002. At the time:

The 2002 procedure began at 7:09 a.m and ended at 7:29 a.m. Bush woke up two minutes later but did not resume his presidential office until 9:24 a.m., after [Doctor] Tubb conducted an overall examination. Tubb said he recommended the additional time to make sure the sedative had no aftereffects.

My question: How could the doctor tell the difference?