I have not found a way to measure the tolerances well.
The hardest part for most people will probably just getting the machine really square. This cnc I made has blown away the expectations I had for it, so I have not pushed it too its limits yet, in speed or precision.
Best I can say is you should have zero worries about tolerances in wood. Metal is where things start to change. In metal the expected tolerances increase dramatically. Not that they can not be obtained, you just need a good understanding of machining to get there. Not necessarily having to even do with the mpcnc.