Im a tank and I know when DPS sucks. Its not this deep level science. There are pretty big tells.

1) Low max HP = low gear
2) Abnormally low Threat levels with no visible use of threat dumps
3) Clearing pulls takes inordinately to long

With a bit of experience, you can pick out whos usually sucking and whos carrying when it comes to DPS. And yes, Ill call out DPS when theyre being abnormally stupid.