The egg will be hot, so don't hold on to it for long.
- Spin the egg.
- Abruptly stop the spinning.
If the egg continues to spin, then it's not completely cooked. In the GIF above, you can see that the egg still spins after I stop it. It's not done yet.
If it stops fully, then you have yourself a fully cooked hard-boiled egg!
In case you're interested...what's actually happening is that you're causing the yolk inside to spin. When you stop the egg, the yolk continues to spin (if uncooked), creating a centripetal force that causes the whole egg to start spinning again.
Want help peeling it? We've got a great trick for perfectly peeled hard-boiled eggs!