For code quality, main thing I did was read every pull request and keep track of how people did, including cases when people improved existing code. I also kept track of times where someone made a contribution to how we thought about quality – e.g. when one person used type-level programming to essentially combine a bunch of tests into one and significantly reduce the amount of code needed.

We had ongoing training – we'd meet every week to go over a chapter of some book – so everyone converged on the same code style which made monitoring quality relatively easy.

With mentoring, I primarily paid attention to what people said about others in terms of mentoring. I also monitored mentoring through code reviews & slack. People's reviews were very consistent – "Bob is the most helpful mentor, Charles is also pretty helpful" – so again, relatively easy in that specific case.