Ask HN: How do you evaluate technical competency for a promotion?

I'm a developer on team with a four-tier hierarchy. We have Associate, Intermediate, Senior and Lead devs. Is there a good (ideally objective) process that a manager can leverage to determine when a particular developer has the technical proficiency of the next tier and would be a candidate for promotion? For the sake of this question, assume the promotion is purely based on technical competency. Management feels they have a handle on the non-technical evaluations.