DBA is very broad profession these days. There is no more only production or development DBA's or just database architects. The companies require jacks-of-all-trades in this position. As a result of that DA become the generalists, which is very negative tendency to the quality of DBA's work and level of expertise.

I am sure the base was there and if you will give a person right structure to work in and time you will see the results. No one knows everything and we tend to forget things that we do not touch time. I bat you have some areas where you are not good.

The task in interview the reveal potentials of the candidate and his strongest sides.

You were trying elevate yourself. You need to get over your memory caused by junior level years and srat to think as a team leader if you want to mentor and get the results of of it. So, your attacks, to the person that is obviously not that great in query tuning anymore, is unfair and shows some level of insecurity in you.

Can you give me in short paragraph the guidelines for the developer to evaluate the query performance without looking at show plan?