https://hrt.github.io/TamperDetectJS/
"It is impossible to determine whether an object is a proxy or not (transparent virtualization)." - https://exploringjs.com/es6/ch_proxies.html
Detecting altered prototypes is useful for when you don't want 3rd party scripts from messing with your page - including anti cheats for certain games. This repo explores methods of detecting if a method has been tampered with as well as methods to get around such detections.
In conclusion, it's possible to determine if a proxy of a native function is a proxy. There are many tells.
index.html is a standalone page with embeded javascript that will check if Array.prototype.join has been tampered with.
You can view index.html on your browser here https://hrt.github.io/TamperDetectJS/
I've included two tampermonkey content script that tampers with Array.prototype.join.
One is a simple proxy that gets detected. The other attempts to conceal the proxy but still fails...
Big thonks to Lemons1337 for chipping in.
Note: This is meant only for chromium based browsers. You will get false detections on others.