...by saying "so what?!"

Either my fears that I'm not good enough to be where I am are founded, in which case I need to be thankful for this opportunity to be "above my station" so to speak, and work hard to make sure I *do* deserve to be there.

...or they're unfounded, in which case I need to work hard to do the best job I can.

In either case, it doesn't change the outcome, so worrying about it is futile. Heck, even go one step further - shoot for the jobs you *think* are above your station, and then see what comes of it.