I know it sounds cliché, but you just KNOW lol. For me, one day I just looked at my partner at the time and he wasn't doing anything particularly amazing but I just started staring and smiling like a retard; and then...I dunno, it's like I was overwhelmed with how happy he made me without doing anything at all and it kinda hit me. I loved him. This is gonna sound sooo corny lol, but it's like I could feel my heart swell *hides face* lol. There's more to it, but that was the initial feeling.