In casual spoken English, people often ask, "Do you know if ___?" to find out the answer to a "Yes" or "No" question. For example:

Do you know if you're having a boy or girl?

Do you know if he's awake?

Answers to "Do you know if...?" can include:

Yes, it is.

Yeah, he is.

No, they're not.

No, I don't think so.

I'm not sure.

No, I don't know.

In formal English for writing and for situations where you don't know people well, you usually use "whether" instead of "if":

Do they know whether the child is a boy or a girl?