Probably the most affordable way to get some confidence is to connect two computers at either end of the cable and do a transfer speed test using iperf or somet

Probably the most affordable way to get some confidence is to connect two computers at either end of the cable and do a transfer speed test using iperf or something like that. If you are using random unmatched computers, do a "control" test with a short, high-quality patch cord between them to get a baseline performance measurement.