It’s more difficult to distinguish if an app is Flutter or some other cross platform framework, but “native or not” is very easy on iOS. Even React Native, which is the least foreign, has tells.

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.

Ah, so we talking about two different things here:

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.

I'm really curious what the specific tells are. Do you have any specific examples?

When using the apps on my iPhone, I just don't see how I could tell the difference.

For React Native it’s common for navigation to be weird compared to UIKit/SwiftUI apps. Also common for things like padding/margins to be off since standard layout facilities aren’t being used, and devs of RN apps will often visually customize controls (web style) that native devs don’t.

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.

There's likely things we missed but we'd love to find examples you might have. We try to be scientific as much as we can https://github.com/flutter/flutter/pull/122275 [coincidentally a community contribution from someone who didn't have to fork :)]

Disclaimer: used to work on some of these animations

Yep, same. For me the animations are a dead giveaway if an app is using flutter. That, and the scrolling just feels _off_ in a way I can't quite describe and is a little clunky.

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.