Heroin and crack cocaine users have their own rituals and it is not that difficult to determine if they are using or not. Heroin users inject the drug and so will have paraphernalia around, as well as track marks on their arms and legs. Watch the behavior.
Is anything wrong, or out of place, or different? Heroin users might only use a couple of times a day, whereas crack users are always seeking the next hit.
Addiction takes over a person's life, so the signs will be there. Just watch. See our section on these drugs and look for the signs of addiction.