You can know if he's interested in exactly the same way you would knowif anyone was interested. Does he look at you a lot? Does he smile when he does this? Does he strike up a conversation with you? Has he asked you out on a date?
My last boyfriend was Japanese and he didn't beat around the bush about it. He came straight out and said he hoped we would get together. The one after that which I didn't go for, took about 4 and half minutes to start talking about being naked...
I guess if someone isn't seeking you out, making conversation and doing all the usual things you'd expect a guy to do he's not into you. If you're really wanting to know for sure ask him out.