Similarly if you currently allow smoking you cannot change the least to prohibit smoking. 

The cost is minimal and I am smoking the closest competitors in my market.

I do feel like anyone serious enough will just take the two minutes to answer the seven questions, which basically just establish if they meet the minimum requirements (3x the income, Credit Score, No Pets/Smoking, etc). 

Try to anticipate other potential avoidable situations with a follow up inspection to assess the Locksmith’s work and ask your tenant(s) if you can also use the time to assess for other issues such as unchanged HVAC filters, leaky pipes, working shutoff valves, working smoke detector batteries, properly working downspouts, clogged dryer vents, evidence of water damage from leaking roof, etc,etc,etc.

In the application, I ask if they smoke or vape.

Always meet in person:  This will give you a good idea of the person, if they're smoking a cigarette while talking to you, or being very negative about their previous landlord make sure you make note of that.4.)

Plus very few of these groups smoke or drink so that's other problems eliminated.  

My suspicion is one tenant might be opening a window to smoke when we clearly stated no smoking.

I would mention to Tenant B that I was visiting Tenant A and smelled what seemed like marijuana smoke coming from their side and remind them there is no smoking permitted inside the house.

The suburbs are smoking hot.