epoll: Distinguishing "listener" FDs

Question

How can I distinguish between "listener" file descriptors and "client" file descriptors?

Here's what I saw in the manpage example:

if(events[n].data.fd == listener) {
    ...
} else {
    ...
}

'But what if I don't have access to listener?

Sorry if this is a vague question. I'm not quite sure how to word it.