Perhaps you'll know when your heart tells you so. Or when it beats against your ribcage with more force than any charge of blood might render.
I don't know.
Maybe you're in love when you find you believe in love at last, even though you had convinced yourself you never would again.
Maybe it's love when you bicker and disagree; but, without fail, come to terms with each other with equal grace and mutuality.
Perhaps you'll know when you laugh together. You'll know even better when you cry together. That takes courage.
You'll know when you're washing up after dinner. When you're lying beside each other in bed, staring at the ceiling, thinking about cabbages and royalty. When you're watching television and the remote is up for grabs. When he opens the door for you, or holds your hand when you cross the road, or lets you walk on the safe side and he on the right. You'll know when he smiles, because he reserves one for you.