Question: What’s one sign a team member is ready to take on a managerial role?

One sign a team member is ready for a managerial role is having a consistent reputation for presenting solutions to problems (not just enumerating issues). Second, they quantify their progress and contribution to the team. Managers are judged based on their perceived progress, so before landing that managerial role, people need to understand the importance of developing SOLUTIONS to problems AND quantifying those solutions.