I deliberately purchased a fake WMO from aliexpress for modding. It had some garbage 1000cpi sensor that would fail at ~0.6m/s, even when 'overclocked' to 1000Hz. The shell colour was also significantly different, and had a different texture on the top shell.
The plastic of the IMO1.1 is usually a good indicator except you'd need an authentic reference. I confirmed I got a fake when the plastic felt almost "fine" and ended up opening the mouse to see that the pcb was made by "MIOROSOFT". I reported it to eBay and will be shipping the two back to China for refund. I will dig for the eBay stores name in my purchase history so that buyers can stay away.
Buy a used or refurbished one and you'll be sure it's original. There are plenty of these in perfect condition because they were used in every other office.