Some pamphlets and websites have lists of signs and symptoms of drug use. They often include changes in school and work attendance, mood swings, changes in appearance, excessive tiredness, lack of appetite etc.
The problem is that many of these things may be caused by things other than drug use and are relatively normal behaviours. Unless you happen to be with someone while they are intoxicated or high on drugs you may not see any clear signs and symptoms. Even behaviour that seems like intoxification could be due to other things.
A danger with following lists of signs and symptoms is that you may jump to the wrong conclusion. If you feel that there is something wrong with a person or that they are acting strangely talk to them about what is going on in their life. Trying to spot signs and symptoms of drug use is no substitute for good communication.