An objectively verifiable indication that an animal has pair-bonded would be a visible indication of distress when forcibly separated from his/her mate. I'm not suggesting that this is the best way to determine whether an animal has pair-bonded. For example, an elevated level of some hormone in the blood stream (a "being in love" hormone) that reliably indicates being pair-bonded would be a superior objectively verifiable indication (in my opinion) because it doesn't involve causing distress in an animal.
I'm not a biologist - just an occasional recreational reader of popular works in biology. So, my opinion isn't worth much.
Right now, it seems that "passionate love" is measured in a discrete scale based on answers to a questionnaire. The "Passionate Love Scale" (PLS) is mentioned in this blog post and was introduced by this article in 1986.
In my other reply to my original comment I showed a study%20Reduced%20cognitive%20control%20in%20passionate%20lovers.pdf) that finds that "high levels of passionate love of individuals in the early stage of a romantic relationship are associated with reduced cognitive control", in which they use the PLS.
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.
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