Love is empathy.
The love between a man and a woman is not some kind of special love that is fundamentally different from other types of love. From the love between mother and child, between friends. It's the same love, just with a slightly different shade. And that's why you can get to know her just like any other love.
For myself, I found the surest sign of love in the Apostle Paul: "Rejoice with those who rejoice, and weep with those who weep." When a person's joy truly becomes my joy, when everything that pleases him pleases me, and his pain becomes my pain, there is love. On a lower level, I would say it's empathy. On a higher, complete level, his life becomes my life, and his feelings become my feelings. Everything that happens to this person concerns me on the deepest level.
How do you know that this is the person with whom you can live your whole life in love?