Hardware info (hwinfo) or similar. Be able to check all voltages, speed and temps while testing new hardware. For example my ARC A770 has little to no info, and shows running at pcie x1.
Edit: mistakingly thought link width was x4, but looking at it again shows x1
Not sure in Linux, could be a driver or kernel configuration. I don't know a way to double check it. When booting into windows it's at x16. So not a hardware or bios issue.