I just received a request with the condition "not from smoking home".  This is not a problem on this occasion since it is the first time the book will be sent.  But, what if I received the book from a "smoking home" and did not realize it.  None that I have received so far smell like smoke to me, but, might to someone else.  How can I know for sure???

You can't.  It's the same thing for my books because I shop thrift stores and yard sales and there's really no knowing.  I'm a smoker and try to keep the two separated but I can't be sure if my nose works as well as a nonsmoker or someone with particular allergies.  With specific conditions I usually apply a sort of "when in doubt say no" policy.  With my own conditions sometimes people take asking for books that are intact as asking for books that are pristine which is a bit of an assumption.  But if it really is loose I understand if they'd feel more comfortable rejecting the request.  It's sort of discretionary, but at least when it comes to conditions regarding smells or a particular quality level and I'm not sure it will satisfy, I decline the request and include in my reason that I hope a more acceptable copy will become available soon.

In general, people who have strong allergies will word the condition differently.  "Not from a smoking home" sounds like an abbreviation of "not currently from a smoking home."  If you get a condition where it doesn't seem clear to you, you can either decline the condition, or you can accept, and then PM the person to clarify.  Your choice. :-)

I have a not from current smoking home and that is what I am looking for, is your home smoking? If not send the book. I think it is obvious that if a secondhand book reeked of smoke you should decline, but if you cannot tell, go ahead and send the book. If I thought a book smelled and marked it RWP and questioned it, all you'd have to do is say "my home is not smoking, I got the book from somewhere else and didn't smell anything" and I would mark the transaction as marked RWP in error.

I agree, there is a difference between 'not from a smoking home' which means that the duration of the time the book has been with you, your home has not housed smokers and "never around smoke" which is a stronger request (I assume for more sensitive allergies).  On the first, I always accept, the second, if I know the history of the book I will accept, but  not if I got it used.

When I get those request.. I smell the book closely and if I don't detect smoke I send it.

No one smokes here but I pick up used books all the time from goodwill, booksales, etc.  I can't GUARANTEE that books have never been around smoke.. but if I can't detect a smell.. I go ahead and send it.