You need to switch into a product manager/QA/UI,UX mindset.

Is what is being built the right thing? Does it work? Is it usable? Has it been built in an appropriate way? (Note this is not a code thing, it’s an architectural question)

Traditionally developers have outsourced these things to other people.

There’s a further thing you should be doing - if the questions above didn’t pass - why not? What went wrong in your process?