This is a short list of things I wrote a couple years ago of things I’d like to see from an engineer who wants to become a manager. This assumes a relatively senior engineer though maybe not the most senior on the team but who wants to move into management. I further assume that they'll have chances to tech lead projects and thus to demonstrate some of these things.
A junior engineer blossoming under your mentorship.
You working well with other engineers at about your same level.
You making good use of more senior engineers on your project teams. This includes being comfortable with someone else being the technical leader of a project.
Your project teams shipping things with everyone reasonably aligned on goals and milestones.
Your project teams shipping things without you doing too much of the heaviest engineering lifting.
Your project teams keeping customers happy even when you can't give them everything they might want.
Think you might want to stick to engineering? I’ve got a list for that too. That not enough? My friend Camille Fournier just posted an excellent list of skills senior engineers need, beyond coding. And a few years back, another friend, Dan McKinley, wrote a very good list of mistakes you apparently just have to make yourself.