This. Without supervision: * can they be trusted to take a story and turn it into a list of one or more problems which are solvable and sensible to solve? *

This. Without supervision: * can they be trusted to take a story and turn it into a list of one or more problems which are solvable and sensible to solve? * given a list of problems, can they be trusted to select appropriate tools for solving each problem in a sane manner? * given a problem and an approach, can they be trusted to fill in an implementation? If you can't answer any of these questions with a firm "yes", they're definitely a junior.