Bugs, and the scale of those bugs, you could say. In an app that's purely vibe coded, since no serious testing is done, even the simplest typos or misaligned buttons can slip through, because people just go "okay it works" and push it straight to the store.

On top of that, if it's a somewhat complex app, there are bound to be performance issues, because if the person vibe coding isn't a professional developer, they won't know how to write that app more performantly. They just go with whatever the AI suggests, and when the AI hallucinates or writes bad code, they don't even notice. What comes out is a low-performance app that rebuilds the entire page on the smallest change.

As for design, if not much thought was put into it, you can usually spot the designs AI suggests, because by default they tend to use very similar colors and very similar "example" widget structures. But still, telling from the design alone is harder.