The Model Inspector allows you to study details about a model's geometry, topology, materials, and textures. Activate it by clicking/tapping the button in the bottom right of the viewer or pressing the I key.
The wireframe view lets you see the mesh's topology (triangles, quads, n-Gons). The keyboard shortcut is 5. If you'd like, you can choose a color from the Model Inspector menu.
You can choose between showing a 3D view of the model, a 2D view of the selected texture, or both side by side.
Final Render and No Post-Processing allow you to enable or disable Post-processing filters. The keyboard shortcut is 1.
You can inspect each material channel that's active on the model (base color, normal map, etc.). The keyboard shortcut is 2.
If you're using 3D+2D or 2D mode and the model has more than one material, you can choose a material in the scene by clicking/tapping it in the viewer or from the drop-down menu in the top-right. If a model has more than one UV set, you can also switch between them from the drop-down menu.
You can enable MatCap mode and MatCap + Surface (if the material has a normap map), as well as show the blank Geometry to better inspect the wireframe, and display vertex normals. The keyboard shortcut to toggle through them is 3 but you can combine the views together with MatCap (6) and Wireframe (5).
The UV Checker lets you inspect the model's UVs using a sample grid texture. The keyboard shortcut is 4.
If you would like to use this UV checker image in your own projects, you can download it here: