By the observable consequences.
Love, or at least some forms of it, can be detected and measured to some degree by the signs of confort, commitment, common goals and mutual support of people. Or you can choose instead to measure physiological effects of people on each other under varied circunstances, detect and describe patterns, experiment with distances, verbal mentions, intentional recalling of absent people. Same for friendship.
It is not all that different from studying electromagnetism (which is very much a mystery when push comes to shove, and more of an evidence of a creator god than anything in emotions or biology IMO), medicine or grammatics.
Concepts, specifically, are human creations and therefore human responsibility. There is no way of measuring their understanding other than by studying human interactions and behavior.