How do you keep up with SEO given how often Google changes, and how do you evaluate advice?

Sources, in order of reliability: Google's own documentation and Search Central blog, Google's own statements from people such as the Search Liaison, then established industry publications, then practitioner testing. Much of what circulates as SEO advice is repeated assertion rather than evidence.

How to evaluate a claim:

• Is there evidence, or just confidence? Ask what was tested, on what sample, and over what period. A single site improving after a change proves nothing — many things change simultaneously.
• Correlation is not causation. Ranking studies routinely find that top-ranking pages share a trait; that does not mean the trait causes ranking.
• Does it align with Google's stated purpose? Tactics that make a page genuinely better for users are durable; tactics that exploit a mechanism are temporary and eventually penalised.
• Test on your own site where you can, ideally on a subset of pages, and measure over enough time to exclude noise.

Note: Being able to name a widely-repeated SEO belief you consider unsupported — keyword density targets, or a specific optimal word count — and explain why, is a strong differentiator. It shows you think rather than repeat.