you cant even tell means you cant identify which is a knockoff "tell” means "distinguish one person or thing from another; perceive the difference between one person or thing and another"

In American slang, a knockoff is an unauthorized copy or imitation of something produced with a more-well known name. It usually refers to designer clothing and is almost always cheaper than the original.

"knockoff" means a fake copy. Example: I just bought a knockoff Gucci bag. --> I just bought a fake copy of a Gucci bag. "You can't even tell" means the same as "You won't know". "I just bought a knockoff Gucci bag and you can't even tell" So if I redo the sentence it would be something like this: -->I just bought a fake copy of a Gucci bag and you won't know that's it's a fake copy because it looks just like the real Gucci bag.

i know that knockoff means "on sale" but maybe in this case it's a british slang meaning "stolen" or something similar... hope that someone else could help us,i'm curious about it! bye