How can you really be sure?

I would say that love is an infatuation that is still there after you really got to know your partner.

"Love is not just looking at each other, it's looking in the same direction." -- Antoine de Saint-Exupéry

I think the first part is infatuation, and the second love.

Love needs time to grow, so the first part is always infatuation. And, does it really matter if you knew which one it is?