Does the rendered UI actually use the design system?

Reading the source tells you what the component asks for. It cannot tell you what the browser resolved: a token overridden three layers up, a hardcoded #3b82f6 that happens to look close, a dark-theme scope that never applied. That answer only exists in the running page.

Reticle reads computed styles in the live app and compares them against the project's tokens. Not set up? Follow the reticle skill: it installs Reticle, asks the user before init changes the app's files, and proves one flow.

The check

reticle_look({ action: "page", sessionId, mode: "interactive" })
reticle_look({ action: "element", sessionId, ref })

inspect returns a theme report per element: