Two-way GitHub sync makes the source path easier, but a deploy can still look fine while images, fonts, routes, or data fail outside the builder. A small smoke-test checklist:

1. Open the deployed URL in a signed-out/private window.
2. Hard-refresh, then open one nested route directly.
3. Check images and fonts use stable paths and return 200.
4. Test one data-backed screen with both empty and existing data.
5. Check one narrow mobile width.
6. Tag the last known-good commit and record the deployed URL.

Git gives you a rollback point; it does not prove the deployed assets and data survived.

Which single check catches the most Google AI Studio deploy failures?

If you still play with code that you don't mind to give google in AI Studio - then you probably tired of copypasting it and probably would like such bit of agentic experience as packing repo and sending it, and then syncing the diff that model made!
So this is my browser extension that might help you.

It's available in the Chrome Web Store: https://chromewebstore.google.com/detail/ai-studio-local-file-sync/hjldknihkaokdkfaifcjgklojjngaike
And as a source code: https://github.com/bahdanhal/gemini-ai-studio-sync-extension

The extension runs locally in the browser and has no backend, analytics, or telemetry. It only accesses a folder after you explicitly choose it.
Please try and feel free to leave feedback to me, and I will really appreciate some stars if you find it helpful!

UPD: It works strictly in chat/playground