Tenant smoking weed but can't prove it

[Chris Kelly]

Good Afternoon. I'm in vermont where it is legal to smoke weed. I have a clause that says you can't smoke anything in the building. There is constantly weed smell coming from this unit. 

Maybe I'm being too nice but I dont want to evict her unless I can prove she is smoking weed or one of her guests are. She wont admit anything and neither are her guests. I am not seeing any smart smoke detectors on the market that can detect weed smoke (freshair only takes large orders for 40 or more) . So I am at a loss and would really appreciate any advice.