Smoking: How to Prove A Tenant Smoking in the House?

[Annie Bollini]

Hi - Would appreciate some advice. 

I bought a fully renovated townhouse, tenants moved in, 6 months ago I saw an ashtray, tenant said she smoked in the basement when it snowed. I wrote/talked to her that in the lease she cannot smoke in the house or would be evicted. She said she would not smoke again. 

A handyman yesterday told me the place smells like someone has smoked. (I will go there in the next day or so to check for myself.) If there are no visible signs of smoking (ash tray, etc) do I have to prove they smoked recently/how do I prove it if I want to evict/tell them to leave?

Thanks