On Android it’s more difficult, partially because it’s kind of like Windows where Google/Microsoft uses 50 separate reimplementations of Material/Fluent and there’s no consistency to be found anywhere.
a) with any given UI design, distinguishing if it's implemented using native UI framework or with Flutter
b) Flutter app providing 100% indentical look&feel to Cupertino/MaterialDesign/WinForms/Cocoa/etc.
I was talking about a). Assuming that the app developer wants to have a consistent app design across platforms, which probably came from a design department – there is virtually no way to distinguish. Ultimately, it's just a bunch of pixels spit out onto the framebuffer.
When using the apps on my iPhone, I just don't see how I could tell the difference.
For Flutter, the most visually obvious thing (aside from usually using Material Design) is that its animation curves are all totally different from those of UIKit/SwiftUI and interact with gestures differently. The Cupertino theme is a poor facsimile of UIKit and lands squarely in the uncanny valley, which is arguably worse than Material Design. Flutter on iOS also tends to hitch where UIKit/SwiftUI don’t.
Disclaimer: used to work on some of these animations
I had been building stuff with flutter for a while when GPay migrated to it and I could straightaway tell from the performance that it was flutter.