Lavender hadn't always known, like some girls do. She'd noticed that she didn't enjoy Seamus kissing her; but she hadn't known why.
When Seamus broke up with her, he said: "If you'd rather be spending time with Parvati than me... it's not worth it." And she felt bad because it was true.
The way Parvati had looked at her that night, asking if Lavender had ever kissed a girl; she noticed that. She hadn't, and Parvati had looked at her with wide-open eyes, and asked if she wanted to try.
When she said yes... well, that was when she knew.