Wow, that's a big question. Most people will tell you that you just "know", but I don't necessarily believe that's true. Our emotions are filtered by perception, so what one person may describe as love is just friendship for others. I know a few people who fall in love about twice a week, but fall out of it just as quickly.
To me, personally, love is the highest level of tolerance, trust and friendship imaginable. It's when you want to be with your partner constantly and think about them as soon as you're separated. You can forgive them over and over, and being with them feels like you're in the right place. It's magic.