Confident my MIL is smoking weed in my apartment

We live on the first floor, with a sliding door for the little patio & walkway.. we only have a one bedroom but circumstances happened & she had to stay with us (her other child kicked her out) so i can basically hear everything she does from the bedroom.  the past couple days i keep smelling weed, just like a draft.. only when were supposed to be sleeping, or my SO is gone.  then moments later i hear the screen close.  now you ask, are you sure its weed.. hell yea because a year before baby i used to smoke heavy & realized i needed to get a better job & you cant do that being a pothead.  so now we have a 3 week old here, & technically allowing her to stay here against the rules.  she helps out the slightest, dishes & cooks here and there but no like money help.  shes been here 3 mos now.  I want to actually catch her in the act but that may not do too much if its not my SO.  Should i just tell him ? or try to catch her first? we cant have drugs here.  shes sick from multiple things but still functional so im feeling like shes probably only doing it to feel better but we can get kicked out for that & were already risking having her here.  My SO hated when i smoked but what if hes like shes doing it to feel better ? i just dont want to seem like a bitch.  we just got in an argument about me not necessarily wanting her here.  we dont have enough space for it & i dont want to risk them evicting us or even fining us cause we dont have the money for that. Suggestions??