pci x16 slot

Solution
Open the case and look. The longer slots in this pic are the x16 slots.

53235-asus-z87-annotated.png

Anonymouselite5

Distinguished
What model board do you have? Usually you can find specs on your board online.
Or you can usually check link speeds in the bios.

If you have a gpu already connected in there, download gpu-z. From there you can see the bus interface.

-good luck