I would give TMs extra tasks to see if they can handle it. For example, I would create captains for certain areas and challenge them to meet their goals. Some I would have run the entire flow process for a floor. I'd show them metrics, why we do things certain ways (Best Practices), shadow a TL, and work multiple areas (Flow, BR, Food process). I'd give them a situation and ask them what they would do. You could tell who had leadership abilities and who needed a little more development. I was blessed with some motivated individuals who should have been at least a TL to start, if not an ETL.