Google 'how do I find the NIC speed on windows X' where X is your version of windows, i.e., if I was running windows 10 I would google 'how do I find the NIC speed on windows 10'. In the event you are running windows 10....
https://winaero.com/blog/see-the-ethernet-or-wifi-adapter-speed-in-windows-10/
As @CCAndrew alluded to, your interface is probably running 100Mb and not 1000Mb or 1Gb (they are the same, it's a matter of reporting). The gigabit standard allows devices to negotiate the link speed. Ideally the two negotiate 1Gb but if that fails, they will then try 100Mb. If they can establish a link @ 100Mb that's where it stays and you'll never see gigabit speeds on that device. If 100Mb fails then they may try 10Mb. The good news is that you can unplug the ethernet cable, plug it back in, and it *may* negotiate 1Gb again.
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