Hammer, king of level editors, has a view mode where the scene is unlit, but not fullbright; it's shaded to make it easier to parse. Unreal doesn't have that! But you can add it yourself, really easily, with no C++ or Blueprint - just a postprocess material and a line in a config file. Here's how!
Here's a pastebin of my material if you want it - just paste that into your material editor.
And that config file text (in my case) was:
[Engine.BufferVisualizationMaterials]
UnlitShaded=(Material="/Game/EditorUtilities/PP_UnlitShaded_Inst.PP_UnlitShaded_Inst", Name=LOCTEXT("UnlitShadedMat", "Shaded Unlit View"))