As I haven't been in love often, I can only answer this from a very limited point of experience. But I noticed that as soon as I become or are incompatible with someone (over time or from the get-go) I won't feel love. It just doesn't happen or it fizzels out. That happened with my first boyfriend. I still liked him, but we weren't on the same page any more and my feelings vanished. His didn't unfortunately, the breakup was a bit ugly.
With Sward, there has been a time of 'incompatibility' but that was more each of us doing our own thing and neglecting the partner over it. It blew up and we fixed it as the feelings were still there. I don't regard that as incompatibility as we just needed to focus on each other again. We weren't realy rubbing each other the wrong way or something like that.
I knew right away that I was compatible with Lin, just because I noticed my feelings grew steadily. If I look back, we haven't even seen each other and we kind of knew that we were compatible nevertheless. Feelings are my one and only indicator to know if I match well with someone or not. But I am afraid, that I won't be a person who works on an incompatible relationship, as my feelings will have decided for me and just left the picture way sooner than my realization that I need to do some work.
I have never been in a relationship that just 'didn't work' for the persons involved or was trying to fix it in vain.
Facts: 30, female, bi, v-type relationship with Sward (husband, straight, mono) and Lin (boyfriend, straight, mono), poly-fi and co-primary.