We can measure it in the release of hormones and watch our minds react to it in MRI. We can see it in our behavior and in the behavior of those who love us. A dozen different people can look at a couple in love and agree, "yup. That's love."
These are all true with respect to deities as well. We can watch brains light up on an MRI when someone prays/meditates/reads scripture; religions (purposefully) influence the way people live their lives; multiple people can credit a deity for something they see or experience.
"I know it when I see it" isn't a good evidential standard, but it's the best one we have for abstract concepts.
I think it's a mistake to allow people claiming the existence of a deity to call it an "abstract concept." At best, they could claim the way you "feel" (experience) a deity is abstract (as are all feelings, hence the question about love), but the deity itself is not. Religions, in general, insist on a specific deity with a specific feature set be worshipped in a specific way, to attain specific benefits and avoid specific punishment. Calling that abstract is a cover, a tactic, a bad faith argument designed to trip people up. It's akin to a strawman, in that it gets people attacking the wrong thing - defining or failing to define love doesn't get anyone any closer to proving or disproving the existence of a deity.