Technically you can’t if they are doing good reviews and modifying whatever LLMs do locally before doing a manual commit. That’s actually a half-reasonable way to use them as well, putting aside the environmental concerns.

But for a vibe coder they are basically just telling an LLM to go ham. The signs will be regular pointless refactoring that touches the entire codebase, big commits, commits happening too fast and in too high of a volume to be actually read and understood by a human, and a focus on style over substance. A vibe coder likes to present that they finished an entire product with a full GUI themselves in a week and tend to write down what it does in the same techbro speak. Look under the hood and it is a mess.

You can also look at how bugs are opened and closed and how long it takes to close them. A vibe coder will have an LLM “fix” an issue on its own very quickly rather than thinking about it or asking for clarification or understanding that it needs to go on a roadmap as a major feature.

You can also review security practices since LLMs seem to be terrible at them.

You can see how many real human contributors seem to be involved. Vibe coders tend to do solo projects, they don’t build a little community of maintainers.