This is a tough situation, and I think others have had a similar dilemma.
Do I tell them that I am poly before there is romantic interest and run the risk of scaring them away or do I find out if there is an interest first.
The problem with waiting is that she may start feeling more and more invested in your relationship, assuming that you are single - when she finds out that you are married and poly she may well feel lied to, even if it is a lie by omission.
The idea that somehow an interest in you would trump an aversion to poly and that this omission of something that fundamental wouldn't be a show-stopper for someone seems problematic to me, especially when poly is supposed to be based on the concept of openness and honesty. What message does it give when the first exposure to poly is that you felt it necessary to hide a significant and relevant aspect from her?
I believe that if poly is a show-stopper, then it's going to be that no matter how well she knows you. So I feel that it's best that you bring it up in casual conversation and have the discussion before turning the focus of your friendship to more romantic topics.
"Listen, or your tongue will make you deaf." - Native American Proverb