We are using CamWorks. Our parts are usually sheets like plywood (except plastic). We need to determine which of the two faces has the most amount of features to cut. This will be the UP face.
We can use the bounding box feature to know the thickness, which narrows down which planes to compare.
I've attached a sample part and an image showing which face should be up. Of course, I use the word "face" when actually there are multiple faces on each plane. You can see that the top and bottom "faces" are split by a groove, therefore, picking the UP face isn't so cut and dry.