I mean anything that can give more information than just the digital bit errors. Like eye diagrams and such. Basically something that can rate the signal quality on some scale wider than "works? yes/no". Bit error rate is typically too low to give a useful indication.

with the setup you're implying, the eye diagram is all you can get, which to be honest you can extract a lot of information about transmitter-side parameters like the jitter, but you can't evaluate receive performance at all.

receive performance evaluation is done by injecting known amounts of jitter and seeing if the receiver can tolerate that jitter.