Five points.

1. Check the theme uses theme.json version 3. Anything older means the theme predates WordPress 6.1 and will miss layout tokens like settings.layout.contentSize. Open the ZIP and read the file. Version is on line two.
2. Verify the theme registers block patterns via /patterns/*.php, not via register_block_pattern() calls in functions.php. The file-based approach is the current-recommended path per the WordPress block editor handbook, and makes pattern reuse cleaner.
3. Check total shipped CSS on the demo. Open DevTools → Network → CSS filter. If a “block theme” ships more than 40 KB of CSS before you add content, it is bundling a page-builder library or a global framework. Real block themes ship 6 to 15 KB.
4. Open the site editor on a fresh install and edit the header template part. If the change survives a theme update, the theme is FSE-native. If the change gets overwritten by a theme-options-panel value, the theme is a classic theme wearing a block-theme label.
5. Read the theme changelog. FSE-native themes list theme.json token changes, template-part edits, and pattern additions. Classic themes with a block-theme veneer list “added new demo importer” and “improved theme options panel” instead.

Sources.

• WordPress block editor handbook: theme.json reference
• WordPress block editor handbook: block patterns
• Rich Tabor: block themes and FSE architecture
• Gutenberg GitHub: theme.json schema (trunk)

Related

• FSE themes for WordPress
• Block themes for WordPress
• WordPress 6.8 changes for theme developers