How can I tell if my sister is smoking weed?!

How can I tell if my sister is smoking weed?

Ok I'm not really asking if marijuana is ok or not, I've done it once before but I don't want my sister doing it. So bascially how can I tell if she's doing it? I understand how I should try to smell her and check her pupils, but I'm almost positive she's doing it at my cousin's house which is far away and she sleeps over there so I can't ever catch her with a high. Is there some type of breathalyzer I can buy thats similar to the ones that cops use for alcohol? Thanks.

3 weeks ago
Maybe all these people on my case are just pot heads who are too lazy to read what I said. I said I "did pot once." I don't smoke and I haven't in about 8 months and when I did, it was my FIRST and ONLY time. Aright, now that we have that out of the way let's move on.

A few of you have brought up some excellent points. I already know she's done it, and she told me she quit, but I think she's lying. She's going to my cousins and she hangs out with a guy and my cousin's boy friend and both guys are the town stoners that everyone buys off of. I guess my additional question is how can I make her not smoke marijuana anymore without damaging our relationship?

THANK YOU to anyone who was helpful.

CHECK OUT HER EYES AND IF HER SLEEPING AND EATING HABITS HAVE CHANGED