How do I know if a guy likes me and how can I smile back?

I like this guy. He is a few years older than me and works at the supermarket where i often go.The first time that I went to his checkout he laughed at something I said, that was a few months ago and ever since then he keeps on looking at me. He glances up and when he sees me he doesn't look away for ages.Its like his eyes can see right through me. He also found an excuse to get really close to me, by reaching around me to get a basket. Another problem is that I find him so attractive that I cant smile back, I just look. I don't want him to think I'm stuck up but I just cant smile. I will go bright pink and then other workers will know that I like him. HELP ME!!!