Accessing navmesh data in code

As I have searched the web, the docs and this forum I can't seem to find the answer to this question; how to access the navmesh data created in the Unity editor in code (C#).
My goal is to use the navmesh data in my A* algorithm to determine which node can be accessed or not. The only thing I want to know is whether there is an navmesh at a give x,z location.
Let me know how or why I can't access this information