iPhone: The cameras and extremely recognizable (to me at least), and most of the time there’s either 2 camera lens diagonal or 3 camera lens also diagonal, Lens #1 being on top of Lens #3.
However, most iPhone models (all the older ones plus the SE) just have one camera at the top left corner.
As for Android, it varies. Some are in the top-middle of the back of the phone (most of the time), others take up the entire back of the phone (talking to you, Xiaomi 13 Ultra (only available in China)). It really depends with Android, but enough camera sense and you can tell that it’s way different than an iPhone.
And that is how I identify who owns an iPhone and who owns an Android.