Don't forget the Poser Materials Room.

I don't much like the colours they use for the display, but it works. Any tool can work better if the mesh is enlarged, Get the viewpoint too close in Poser, and the selection stops working.

There's a button for reversing normals, and you can select individual facets. You need to set up a temporary New Group, pick the facets you want to change, and reverse the normals. Then delete the group.

You can add a material to a New Group, and reverse the whole material, then deselect facets facing the correct way, and reverse normals again.

The general problem is that the file can contain specific normal data, which Poser ignores. Poser derives the normal from the order of the vertices in the file when the facet is defined, Two different rules, and sometimes different results.