How can I differentiate between the unmarked PCIE slots on my motherboard?

"Without powering up the system, you can most likely tell from visual inspection which slot is x8 and which is x4. Each PCIe pair of traces (differential pair) has two series capacitors (AC coupling capacitors). The capacitors will be lined up on the top and/or bottom in pairs parallel to the long edge of the slot starting closest to the interior of the board. Given that the manufacturer won't populate the capacitors on the unused traces to save money, you'll see 4 pairs of capacitors for a x4 slot and 8 pairs of capacitors for a x8 slot.

Here is a picture of the an example CAD layout of one set I found through a Google image search. The rectangular pads are where the capacitors would be populated:

You can easily see the AC coupling capacitors on either side of the top slot in this picture:"

"You can always check CPU-Z to tell you which is which. Or you can use SIW.

Both of them should give you a good look at your board. I think that SIW should be better thatn CPU-Z.

-JFV"