I can't tell if she is interested in me or not... help?

there's this girl at my school who Ive liked for a while (3 years) and i kinda think that she knows i like her a lot. i wanna ask her out, but i wanna make sure that she even likes me so i don't seem like a dumb if she doesn't. i have no idea how to tell if she likes me or not cuz she is always so discrete and un-obvious about everything