Tell me some truly objective methods to find out if someone is skilled. There is always a subjective reason why someone can fail. Whiteboards? Some don't perf

Tell me some truly objective methods to find out if someone is skilled. There is always a subjective reason why someone can fail. Whiteboards? Some don't perform well with those. Take-home assignments? People can lookup solutions. Everything can be used. Asking someone: what did you learn in the last year? How? And what did you gain out of it? These don't seem hard and unfair questions to ask.