Hoe do you preemptively figure that out? You don't get romantically involved with a person until you've known them long enough for the infatuation to have worn off and you've interacted with them in a wide variety of situations over many months.
I've one friend with whom there were sparks, though when we first met we weren't in a situation conducive to us getting together (her mono marriage was a major barrier). By the time we were in a situation where we could get together, I'd figured out that we weren't compatible as a primary couple. As much as I love her, we couldn't coexist that closely without some regular conflict. As she seems to be mono, I'd have to be primary with her and it just wouldn't be good, so we have a friendship.
The only thing that allowed me to figure that out was the time involved from meeting to when we could have gotten together. It likely would have taken the same amount of time to figure out had we just hooked up at the beginning. The difference is that the one situation involves an ongoing friendship while the other would have involved an ended romance.
So, if you meet somebody new and interesting, I figure you can figure out whether you're compatible for the long term by spending time assessing that. If you're more afraid of not being compatible than you're optimistic about it working out, be friends for a couple of years first. Otherwise begin dating to figure it out.
When speaking of various forms of non-monogamy...it ain't poly if you're just fucking around.
While polyamory, open relationships, and swinging are all distinctly different approaches to non-monogamy, they are not mutually exlusive. Folks can, and some do, engage in more than one of them at a time--and it's all good.