Keep in mind, DUI stands for "Driving Under the Influence" which includes alcohol and other drugs. DREs have various ways of recognizing other drug use. An easy way to spot users of some drugs is to measure pupil size. The diameter of the iris (the colored part) is about 12 millimeters (mm). The pupil (the dark part) is 2.5 to 6.5 mm in normal adults (slightly larger in children) depending on light conditions, or about 1/5 to a little over 1/2 the diameter of the iris. If it's less than 2.5 mm, "pinpoint" sized, opiate (e.g., heroin) or non-therapeutic levels of opioid (e.g., Demerol) use is evident. There is evidence of marijuana or heavy alcohol use at 6-7 mm in light (these drugs can also cause the eyes to appear red or "glassy".)
"Does he often have red eyes, flushed face or 'drinker's nose'?" This affects a select few and only with certain drugs. It's important to recognize that some addicts' eyes never turn red, while the eyes of some non-addicts do so with as little as .04 BAC. I, personally, suffer from facial flush including red eyes at low levels, very discomfiting indeed. Some Asians, who experience this due to the lack of a gene to properly process alcohol, are reported to have a very low incidence of alcoholism. Even if the predisposition to addiction otherwise exists in such individuals, they may not continue drinking due to the discomfort experience. However, some continue to use despite this condition.
"Does he often have glassy eyes?" Check out Marilyn Monroe's bedroom eyes in her movies. She looks sexy with those glassy eyes. We now know she was drinking on the job. Aside from alcohol, marijuana causes glassy, red eyes, while stimulants can result in a glazed or tired look, due to lack of sleep.
"Does he often have dilated or constricted pupils?" As was previously discussed, pupils less than a fifth in diameter of the iris are indicative of opiate use. Slightly over half the size of the iris in the light signals heavy alcohol or marijuana use. Anything larger in adults is an almost certain sign of non-caffeine stimulants in non-therapeutic doses.
While this is a wonderful tool for identifying non-tolerant and occasional use, according to addiction expert Forest Tennant, M.D. most addicted (tolerant) users exhibit normal pupil size. Furthermore, such users may fail to show any of the physical signs of substance use and being under the influence. Unfortunately, the only unequivocal physical proof of use in many is blood or urine testing. This is the reason we must look to behavior patterns in identifying substance addiction, using physical signs (when available) only for supplemental verification.