I'm on console, so I can't really measure a fellow DPS's performance very accurately.
If I feel stuff melting fast, I commend the other DPS and even sometimes praise their performance at the end of the duty.
If they can outDPS my BRD (or gives me a run for my money) while having a not-so-stellar gear, they definitely get a comm.
If I notice them using their different tools well, I commend them.
If they're friendly and aren't atrocious at their job, I give them a comm too.