The motherboard in question is the ASUS P5MT-M Micro ATX
The following image shows a couple sections of information taken from the official ASUS manual for this motherboard, along with an image of the entire motherboard taken from the ASUS website. (The motherboard in the image is REV:1.01, whereas mine is REV:1.03 - I don't think this matters too much for the question at hand.)
The PCIE slots are not identical - one has an x8 link and one has an x4 link. However, they are both x8 size to make this micro motherboard slightly more robust. They appear identical visually - and as you can see from the image of the motherboard, every square millimeter of this motherboard is crammed with a chip or a capacitor or something. There's absolutely no room for component labels - or anything helpful at all, really.
What I need to figure out is which one of the PCIE slots is x8 and which one is x4 - in order to maximize performance of PCIE expansion cards. I have provided a link to the most recent official manual from ASUS - but I can't find anything in there that says which slot is which. Does anyone know where this information might be found, or how I might figure it out for myself?