This sometimes takes some real detective work.

Is there a way to detect that before buying it?

As little as .1% is detectable and that is below the threshold for infection.

.- Not willing to add yet a recurring cost-----------------With the above I went with Wyzecam, a recent startup building cute little cameras, wifi connected, 1080, night vision, extremely clear pictures, real-time stream, motion detecting with sensitivity adjustment, sound detection, 24-hour sliding recordings, all for $20 with no subscription needed.

Whether they do a convincing enough job to avoid detection or question of impropriety or the ability to mount a good defense in the case of a prosecution is a different question. 

The reason is, a person way off in the distance doesn't move enough to trigger motion detection, and you'll want to know what they are doing in that back corder for 5+ minutes.

But DO NOT shell out for any mentoring programs or other things until you've been attending for awhile and your BS-detection skills have been honed.

It does have the ability to trigger responses (e.g. if it detects you have a new booking it can automatically send a thanks-for-booking message), but I don't use that feature - instead I have it set up where it'll automatically send the standard messages (ones I've customized) to my guests at times I set.  

So see if you can detect the smell the next day.

If you handle your showings yourself, when they arrive and come inside you’ll be able to detect the smell of smoking on their clothes.Nothing illegal about that.