A rockstar developer doesn’t churn out unmaintainable, by definition.

The number of people who think they are rockstar developers but clearly aren’t is probably close to the number of cover bands who see themselves as undiscovered rockstars.

I’ve worked with people like this, their best hope is to fail upwards into management.

The only way to know if you are competent coder is for other coders to tell you. If none are telling you, your imposter syndrome isn’t.

There are other signs as well but these aren’t taught in formal education. An example being the ability to recognize how your old code could be improved. The way requirements stack over time makes this a certainty in any product.