While most components (cooling body, heatblock, heater cartridges, thermistors) of knockoff e3d-v6 hotends can be similar enough to not notice in a large part, or at least functionally the same. Note that I don't say the parts are necessarily interchangeable - a lot what is sold under the name is not what it claims to be. The main distinguishing difference is the heatbreak: a proper e3d v6 hotend is hard to manufacture, so let's compare the designs of the real and the knockoffs.
e3D v6 heatbreaks are necked considerably and have a socket with an airgap to the cooling body in top to take the lining and prevent heat flow to the PTFE pipe. This makes it an "all-metal" hotend, because the liner stops in the cold zone, where the cooling block is screwed on, and thus gets the least heat to the bowden, allowing to print hotter than with a lined hotend. The cooling block and the heather block threads are also not the same: M6 and M7. They are flat at the front to securely butt the nozzle thread against.
Their internal and external geometry looks like this:
Knockoffs vary a lot how their heatbreak is constructed. Some are lined all the way through. Others are pass-through - and neither is an all-metal hotend. Some knockoffs are necked but use the same thread on both sides.
gallery of knockoffs
If you print a lot of high-temperature materials, the need for a proper all-metal hotend is there. Many knockoffs don't do this necking properly or invalidate its effect by lining the hotend all the way through.
Note that the machining quality can be quite different.
Due to sizing, not all knockoffs allow to mount a genuine heatbreak with knockoff cooler and heater block.