How do you spot a dev or engineer who is ready for management?

You don't spot them, you ask them! 

And you don't just promote them, you give them the skills you need first.

Organisations fail when they promote people before they are ready

It's a two way process, there should be intent from a developer who wants to take the management path, and the second being another spotting those traits.
Management is leadership, so all traits of leadership - empathy, teamwork, communication, accountability, strategic thinking, self motivation gets that tick.

A few things that may be good indicators:
* Care for others and can build good rapport within the team and also outside the team.
* Big picture thinking, not always thinking at the task level.
* Able to prioritize with good trade-off analysis.
* Care for how to unblock the team vs only how to unblock self.
* Taking interest in management/leadership through upskilling, mentoring, etc.
* Dare to raise important issues that others do not think about or care to talk about.