Unfortunately, it tends to be a bit "squishy" - task completion (by the PE or by junior team members), team velocity, etc can all be used, but using them exclusively is just asking for the team to game those metrics. Much better to have regular 1-on-1s with each person and just discuss progress - team progress, individual project/task progress, and any career goals. If somebody is actively seeking promotion, we'll use a job progression matrix (HR puts them together) and talk through the differentiators and what that employee can do to fill the gaps.