How to tell if my mom smokes?

So I've caught my mom smoking a few times in the last 3 months or so. Every time I tell her to stop and she says she will but then I catch her again. In the last 2 months I haven't seen her smoke and I thought she stopped, even though the house would sometimes smell like smoke and I'd see cigarette packs lying around (and I even saw an ash tray with a bunch of old cigarettes and a lighter on the window next to her bed) but she'd always blame it on her boyfriend, who smokes. Well I was playing on her phone recently and she got a text from her boyfriend, so I opened it and was about to hand her the phone and tell her she got a text when I saw a text a little over a month ago at the top of the screen that said "I've been feeling really stressed... So I got a pack of smokes ;)" Now I'm seriously thinking she might still smoke, but I'm not sure. How can I tell if she's smoking and are there any ways I can like spy on her?