Now that’s a different question, and it depends on which aspect of love you mean. We can’t use my wife’s internal subjective feeling as evidence for much of anything for the reasons I’ve stated above. She can say that she has those feelings, certainly, but we can’t know if those feelings are what we call love, or that they are anywhere close to what other people experience when they describe their feelings as love, just like we don’t know if her internal subjective experience of blue is the same as anyone else’s.
But the external manifestations of love can be confirmed by objective evidence: her statements, her behavior, her subtle body language, etc. Those externalities might make us think we can infer what her internal subjective experience is, but only to the extent that we can say that my blue is the same as her blue.