It is possible to download and use UEFITool, and that can tell whether your mobo UEFI supports bifurcation. AMD provide a guide on how you can do that with an AMD motherboard:
The tool tells whether the UEFI supports: x8/x8, or x8/x4/x4 or x4/x4/x4/x4m, and PCIE 4x4 can be found hidden behind...