I have just UV unwrapped my model and I will be editing its texture in Photoshop but it's hard to tell where each things are and I have to do a lot of trial and error. I know you can generate a grid texture as well (Color Grid and UV Grid) which is more helpful but still not what I have in mind.
What I want is to generate a texture where each face is colored differently to help me figure out exactly what is what in my image editor. Does blender have a feature to do it automatically?
There is an addon called Export UV layout that allows you to export your UV unwrap layout as vertex and edges so you can draw over them in whatever graphics editing software you use. Not sure if that is enough or what you were looking for.
If that is not enough you could assign different materials to your object's faces with different colors as desired, bake it to a texture using your existing UV layout and the export that baked texture to Photoshop so you can paint/edit over it.
Alternatively you may also play around with cycles procedural materials, and create a setup that highlights your geometry details in an informative way (the geometry input node as some useful options) and then bake the resulting shader to a texture using the UV layout.
Unwrap your mesh. Note that resulting image of colored faces will depend on this step, so you may want to unwrap with seams in order to get appropriate islands.
To make addon color individual faces instead of whole object you should make each face being individual. This can be done by extruding individual faces and deleting original ones.
Enter Edit mode of the mesh, select all and press Alt+E > Individual Faces. Cancel moving of extruded parts by pressing Esc.
Invert selection with Ctrl+I to select only original faces and delete them.
Enter Vertex Paint mode, in Paint menu find Add random vertex colors to connected vertices operator which is what addon is. After executing it the faces will be colored randomly.
With all selected choose W > Remove doubles; individual faces are no longer needed.
In materials node of the object add Emission node (I chose it to see clearly what the colors are when rendered; it could be Diffuse shader as well, just when baking choose to bake Diffuse Color) and Vertex Colors > vertex color name (by default Col). Connect them appropriately.
Image Texture node left unconnected intentionally; see below for the bake details.