I would be careful applying this one to imposter syndrome. I think of imposter syndrome as a failure to calibrate your expectations of yourself with the reality of what your work requires. Asking yourself what the "real" person would do that you aren't doing seems to me like it would play into that miscalibration. You might be better of asking _someone else_ what your replacement would do.
Relationship is failing (and you want to save it): what will their next partner do differently? Do it now!
Worried you might be kicked off a sports team: what will the new player do better than you? Practice that!
Imposter syndrome has you worried your boss will fire you: what will the replacement, the "real" person do that you aren't? Start doing that today.