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It's "easy". You just spend a couple of years reviewing PRs and working in a professional environment getting feedback from your peers and experience the consequences of code.

There is no shortcut unfortunately.

You improve this skill by not using LLMs more and getting more experienced as a programmer yourself. Spotting problems during review comes from experience, from having learned the lessons, knowing the codebase and libraries used etc.

Find another developer and pair/work together on a project. It doesn't need to be serious, but you should organize it like it is. So, a breakdown of tasks needed to accomplish the goal first. And then many pull requests into the source that can be peer reviewed.