How do you find competitor backlinks?

On zero budget: ahrefs and semrush both have free backlink checkers that show a limited list for any domain, which is enough for a competitor's top links. Moz has a free tier as well. Then the useful bit is the gap. Put two or three competitors through it, note the sites linking to all of them but not you. Those are your shortlist. Expect months, not weeks, before links you earn show in rankings.