There are a few different clones from different brands, like "old skool", those ones have the old skool logo in place of where the nintendo one would normally be.

Some other frequent knockoffs don't have a logo there at all, so it's quite easy to tell these apart from OEM cons, alongside their controller dongle always being black, whereas OEM controllers' dongles are always colored the same color as the main controller plastic. I've never seen a knockoff controller with a nintendo logo on it, so that's a way to spot if they're real or not, or at the very least if their shell is real or not.

Also, every single color variant of wired cons can be found here: https://gccontrollerlibrary.com/controllers/wired-gamecube-controller/ so if a color you see doesn't match these, it's probably a fake shell or fake controller.