Well the first give away is in appearance. MXE is a very fluffy white powder (stuff I sampled anyway) where as K should be crystals, I've never bought it as powder but it does happen when some idiots decide to cut their gear. (don't buy it powdered)
Secondly, ketamine will numb your gums rather well when taste testing, MXE from memory doesn't but I could be mistaken.
The easiest way to tell is the duration. Ketamine is a very quick up-down with a rapid onset. MXE takes foreverrrrrrrrrrr to fully kick in and when it does it lasts many hours with lingering effects into the next day, K just tends to wear off after an hour or two. Also motor function on Ketamine is very limited, dose dependent of course. MXE you can still move with relative ease.
Ketamine also has opiate activity, but MXE seems to be significantly more active on that front.
I doubt people will ever try an pass off MXE as ketamine to sell, but there are some fools out there. Potency of MXE is easily double that of K imho. Some "acquaintances" helped themselves to a bag of MXE belonging to my friend on the way to a doof recently thinking it was K. Needless to say they got alot more than they expected racking up K sized lines