The people I know that have fallen into this trap tend to have a lot of hubris and arrogance. They probably couldn't even fathom the notion that the label might be applicable. If you're even asking the question, I'd say there's a good chance that you're safe.

More practically: As stated above, if you think you're the most competent person in the room, start looking for other rooms. You don't necessarily have to find new employment to do this—attending meetups and conferences, doing a side project with others, etc. can provide a barometer of where you stand among a wider community.

I propose to study technical literature. Study advanced books at a university library. If what is written inside is new to yourself AND you consider yourself an expert, you are in the expert-beginner pit.

Well, I do study technical literature — just today finished reading lecture slides for Stanford's CS143. The problem is, the technical literature that's interesting to me is dealing with much harder problems that I get to solve on day-by-day basis.

You can't know everything there's far too much tech to keep up with; so everyone's expertise is some kind of local optimum. The question is just how local? i.e. How narrow is the pit that you're in? How filtered are the ideas that you receive?

So looking a bit wider, e.g. "meetups and conferences" if you don't already attend, might help.

Beyond that, look at people working in parallel programming languages - What is the main alternative to the languages that you use daily? And how do the people there do things? In what ways is it better?

Maybe: have you recently questioned whether doing X is the best approach for a given problem,and possibly investigated alternatives or sought input from others? For 'simple' things this still applies, even if your day job has unsexy issues.