For me it start with feedback from relay users in the first place. I really like the concept how Coracle has integrated reviews for relays for example. This is done with notes kind 1985 (a label) from NIP-32.
After this feedback compoment comes the performance (which are aggregated on https://nostr.watch for example). You could combine those two aspects to decide if the relay is giving a good service.
good servicestands for?"good service.