How to tell if you are incompetent?
It's way easier to point out to competence than incompetence. I would use as a prime measure of competence finishing things (projects, software, assignments). While failure to complete things can be due to various facts (including the possibility that the deliverable was impossible to complete in the first place) completion, delivering, is a clear, logical measure.
So, to me, true artists ship, shipping is a good measure of competence but lack of deliverables does not prove incompetence, it just means lack of evidence for competence.
"Incompetent" as a word has a really nasty sound to it. Also, people learn, so several failed projects can actually lead eventually to a success and delivery, if the person is able and willing.