The best method is still competitor backlink research. Check where competitors are getting guest posts from, because those sites already accept placements in your niche. That usually saves a lot of time. Then manually qualify every site by checking its real organic traffic, niche relevance, backlink quality, outbound link patterns, and whether it’s actively publishing fresh content.
I also look for sites with multiple real authors and actual engagement. Most “write for us” lists are outdated now, the best opportunities usually come from manual outreach and relationship building.