They've printed jet engines with it that work after polishing. So how is the accuracy measured and how accurate is the DMLS process? I can't find the required info anywhere.
Accuracy will vary a lot depending on what you are interested in.
At the simplest level you are limited by the layer thickness, which is in the $20-100~\mu m$ range. This places a limit on the accuracy of the dimensions that can be achieved in the z direction. Theoretically you can achieve very high accuracy in x and y, but in reality you are limited by the roughness of the the part.
Indeed, the real problem with most of these additive technologies is that the resulting surfaces are very rough, hence why polishing is needed to finish the surface. I don't know for DMLS specifically, but for related methods, such as SLM, $30~\mu m$ roughness would not be bad. Generally, the roughness is highest where the part is angled with respect to the layers as you then can get additional effects from going over steps.