When opening certain .dgn files the screen is black, no objects visible - but I can see all the levels of the model in the selection tree - how do I tell if it's a bad file or if there is something wrong with my visibility?
The first thing to check would be the Scene Statistics for the imported file. Does that show geometry being converted & suitable nodes being added that correspond to solids in the model? If not, then there's no geometry being converted in your model. Otherwise there are a few things you can check.
I would recommend changing the background colour away from black (the option should be available on the Ribbon), or selecting items in the selection tree. The selected items should highlight with the default highlighting colour. A combination of these should indicate if anything is visible.
Two other things to check are whether the items are coming in as hidden (in which case you can just Unhide them) and whether your default view is for some reason facing away from the actual model.
If all of those fail, you should be able to Show Developer Properties under Interface/Developer in the Global Options (this may require the Advanced Global Options, accessed by pressing Shift whilst clicking on Options), and see if you have any Geometry properties present in the model, and if so, what they are. If none are present, your model has no geometry.