How do you approach a guy at work?

Hi there,

So I'm an English teacher working at a school in Thailand and I have a crush on this other (thai) teacher.

He works at the office above me so I never really have a chance to talk to him unless I go to his office for work reasons. He usually eats in his office too and I just smile at him whenever I see him. We've talked a few times but there were always people around. He smiles back at me and he's helped me once with paperwork but I want to get to know him better. I have no idea whether he has a girlfriend and I don't want my coworkers to know I have a crush on him. Dating someone from the school wouldn't be a taboo here as long as you avoid PDA but how do I find a reason to talk to him? And how would I know if he's even interested?

Thanks!