I guess what I really need to know, is what specific questions should I be asking to make sure they really mean it when they say they can be poly-compatible? Outside of determining if they've had experience being poly before, how can I tell if someone is seriously interested in being poly or poly-compatible?
A lot of people go into relationships thinking they can "change" the other person. This happens quite a bit when it comes to having children - one partner wants them and the other partner doesn't, and each thinks the other will eventually change their mind.
One thing you can do is not to ask questions, but simply TELL the other person that this is the way you are and you do not see yourself changing. Do this at the beginning and have a conversation about it every now and then when you're both in a good mood. Keep up the conversations about poly and ask how the other partner feels about it at whatever stage of the relationship you are in... ask if they have met anyone else they find interesting, etc. Make this a regular thing so it's not like a heavy, "let's sit down and discuss our relationship" type of thing. And make sure you do this all the time, not ONLY when there is a problem or when one of you has met someone else.
I think the key to good communication is to communicate pro-actively and not to wait until there is a crisis. Get yourself used to communicating effectively when there is not an issue to resolve. Check in with each other and find out what's going on in each others' minds and lives.
Having said all that, you can only control what you do, not what others do. If you are with a partner who is delusional, they will tell you what you want to hear and still do whatever is on their agenda.