I'm usually a Myers-Briggs 'T', which means I filter my feelings through my thinking, but when it comes to "have I fallen in love?" I'm not a 'T' at all. I don't ask, I just know.
What follows is a poor attempt at putting the knowledge (which is not at all verbal) into words.
I know I love someone when I feel whole and perfect and complete when I'm with them. Sometimes time stops when we are together.
I don't feel that every second I'm with them, but it's always there underneath, it just gets covered up by other more superficial stuff sometimes.
If they feel the same way about me, and we get to spend time together, then it's a love relationship.