You check domain authority using third-party SEO tools, not Google directly, because DA isn't a ranking factor Google uses. The three main sources are Moz Link Explorer for Domain Authority (0-100 scale), Ahrefs Site Explorer for Domain Rating, and Semrush Domain Overview for Authority Score. Moz coined the term and their DA is most widely cited in link prospecting, but all three metrics predict ranking ability by evaluating backlink profiles. Moz offers limited free checks per month before requiring a subscription. Ahrefs and Semrush operate similarly, showing DA/DR/AS alongside backlink counts, referring domains, and organic traffic estimates. For quick free checks, tools like Moz's browser extension or websites like websiteseochecker.com pull basic DA data, though they're less reliable than direct tool access. Domain authority matters most when comparing prospects during link building. A DA 45 site isn't necessarily better than DA 38—you need context about niche relevance, traffic quality, and link placement. At Ottawa SEO, we typically pursue DA 30+ for local Canadian clients and DA 50+ for competitive national verticals, but a DA 25 niche blog with engaged readers often outperforms a DA 60 content farm. The scores update monthly as link graphs change. Don't obsess over your own site's DA fluctuating 2-3 points; focus instead on the trend over six months. A drop usually signals lost backlinks or competitors gaining links faster than you. Also remember these are predictive metrics from private companies reverse-engineering Google's algorithm, not direct ranking inputs. A site can rank well with mediocre DA if it has strong topical authority and user signals. For portfolio management across 500+ domains, we batch-check DA through API integrations with Ahrefs and Semrush, flagging domains below expected thresholds for link audits. Manual spot-checks work fine for agencies managing 5-20 client sites.