How do I detect player motion cheating?
It is up to you to decide what kinds of cheat detection your game requires.
The server has a complete representation of the world geometry that is the same as is available to the client and you can perform whatever validity checks you feel are necessary for your game. In practice, most MMOs work under a trust and occasionally verify paradigms as a compromise between optimal server performance while detecting cheating over time, with perhaps algorithms that escalate the frequency of cheat checks for a particular client that has previously exhibited abnormal behavior. If your game requires it, you could validate (asynchronously) every movement the client makes...but this has a processing cost you have to determine to be worthwhile for your product versus a less frequent detection strategy.