If a person exhibits one, or even several of these signs, it does not necessarily mean that the person is intoxicated. Change in normal baseline behavior should be considered.
Physical signs
1. Loud speech, slurred speech, changes in tone of voice
2. Bragging
3. Overly animated, entertaining, friendly
4. Peculiar odors of marijuana, alcohol, solvents on body, clothing, in the house or car
5. Presence of drug paraphernalia in environment or dirty laundry (seeds, leaves, butts in pockets)
6. Use of small baggies, cigarette papers, unusual small containers, scales
7. Use of eye drops, room deodorizers, or incense
8. Makes inappropriate comments about others
9. Bad behavior, swearing
10. Inappropriate sexual advances
11. Says things that do not make sense
12. Weight loss
13. Drowsy, falling asleep
14. Bloodshot, glassy eyes, change in pupil size
15. Lack of focus and eye contact
16. Clumsy, uncoordinated, stumbling, falling down, bumping into/dropping things
17. Decline in physical appearance (lack of alertness, decrease in neatness or personal hygiene)
Environmental Clues
18. Missing money or property, selling of personal items
19. Change in school performance (drop in grades, attendance, school disciplinary problems)
20. Keeping secrets about friends or activities, change in friendships
21. Lying and denying
22. Obnoxious or mean
23. Changes in mood (aggressive anger, uncaring attitude, crying, depressed, argumentative)
24. Major changes in behavior and personality (loss of motivation or interest)
25. Difficulty concentrating, understanding, remembering
26. Rambling conversation, loss of train of thought, slow responses to questions
27. Change in family relations (withdrawal, hostility, marked increase in family arguments)
28. Difficulty following rules/limits
29. Involvement in illegal activities or delinquent behavior
30. Refusal to take a drug test or be tested for alcohol use
Commonly Abused Drugs (signs)
Depressants: (including Xanax, Valium, GHB): Contracted pupils; drunk-like state; difficulty concentrating; clumsiness; poor judgment; slurred speech; sleepiness.
Hallucinogens: (LSD, PCP): Dilated pupils; bizarre and irrational behavior including paranoia, aggression, hallucinations; mood swings; detachment from people; absorption with self or other objects, slurred speech; confusion.
Heroin: Contracted pupils; no response of pupils to light; needle marks; sleeping at unusual times; sweating; vomiting; coughing, sniffling; twitching; loss of appetite.
Inhalants: (glues, aerosols, vapors): Watery eyes; impaired vision, memory and thought; secretions from the nose or rashes around the nose and mouth; headaches and nausea; appearance of intoxication; drowsiness; poor muscle control; changes in appetite; anxiety; irritability.
Marijuana: Glassy, red eyes; loud talking, inappropriate laughter followed by sleepiness; loss of interest, motivation; weight gain or loss; excessive snacking or eating at inappropriate times.
Stimulants: (including amphetamines, cocaine, crystal meth): Dilated pupils; hyperactivity; euphoria; irritability; anxiety; excessive talking followed by depression or excessive sleeping at odd times; may go long periods of time without eating; weight loss; dry mouth and nose. http://drug.addictionblog.org/how-to-tell-if-someone-is-on-drugs/