How do SEO audit a website?

SEO Audit Basics—The Things You 100% Must-Check

1. Benchmark Your Rankings and Understand Your Competitors.
2. Check for Duplicate Versions of Your Site in Google's Index.
3. Check Your Site's Indexed URLs.
4. Check for Manual Actions.
5. Analyze Your Site's Speed.
6. Confirm That Your Site Uses HTTPS.
7. Check for Mobile-Friendliness Issues.

Those potential areas of issue that could prevent your site from being properly crawled and indexed or serious issues that could be causing areas of concern.