Got this crush on a girl in their mid to late 20s

She and I have met once in passing. Eye contact several times and a brief "hi" with what I thought was a twinkle of attraction in her eyes.

So my plan of course is to show up again and just start talking to her.

My challenge is that although I'm okay talking to pretty girls, I tend to assume they're just being polite when they laugh and smile while we chat.

So question is: how do I read "romantic interest"?

Thanks.