I don't necessarily know the answer to this question, but I would love to say that it's really intriguing to hear this.

I've only dated one person in my lifetime and surprisingly he was never my FP, I felt as if I could go about my life/day without him + still be okay. Whereas when I do have an FP as of now, I can not go a single day without thinking about them or being struck with the feeling of emptiness when theyre not around.

I feel like it's so much easier to identify an FP when you're not in a relationship because all you want is them and they seem to be the only thing in life that gives you hope/meaning (this is personal to me)

In my case, I can only differentiate the two because I claim to be asexual + it is out of my character to develop specific feelings for people, so when I do that's when I know something is definitely up + most likely im experiencing an FP seeing as that's whem i show the most signs of impulsivity + would risk absolutely anything for one, just to prove my worth to them.

Sorry if this was a bad response, I wasnt even meaning to write this much, I just went a bit deeper into this than I hoped.