not quite sure here but if you noticed an unexplained changes in behavior or physical appearance then most probably they are using drugs. also a change in appetite.

here are some physical changes of people who are using drugs..
 - poor hygiene
 - bloodshot eyes
 - change in sleeping patterns
 - agitated speech
 - sick more frequently

one thing to know that your kid has been using drug is when he/she has been acting strange and not normal. he /she may also act violently.

if he/she has been hanging out with friends more than the usual. he/she may also have a change of sleeping patterns.

i suspect that my son has been using drugs because lately he's been cutting classes and had very low grades. what do you think?

well probably he/she's been using drugs but it's hard to say not unless you have evidence of him/her doing it.