Is there a sensible way to measure how well a team is self-organising?
The most critical measure is whether or not the team can deliver Done work. Without that, any assessment of their ability to self-organize is meaningless.
Who makes decisions on what the team does and how they do it? If there is 1-2 people making the decisions, then maybe not. But that isn't a set rule because a self-organising team could choose to organise in a manner where 1-2 people lead the team. Self-organising is centered on the people involved making the decisions. One thing I have always used for myself is to question whether the development manager(because let's be honest, there is always one of these), Product Owner, Scrum Master or most senior team member can be gone for an extended period of time and the team will carry on normally. In fact I encourage that to occur just for this purpose. It also helps the team realize their own capabilities.
@Ian Mitchell said, self-organisation is just a means to an end. The real measure of success is the ability to consistently deliver Done increments of value that the stakeholders appreciate.