It kind of depends on where you are in residency. For upper level residents it's pretty obvious (and we have in-service exam scores to back that assessment up). But residency is about ability to "do" as much as it is the ability to "know" and the former is usually the bigger challenge early on... basically having to run through a mental flowchart of "what do I do in this situation?" is the skill that matters most early on in whether or not you make it as a resident.
Even well-read interns are going to have A LOT that they don't know yet, so getting up to speed on figuring out what you do know, what you don't, and who and what to do when you don't have the answer is the sink or swim thing early on.