Mainly by the mistakes they do, or if they actually listen what needs to be done.

There's some DPS that like to tank, they throw themselves at mobs... There's the tanks that can't keep aggro... There's the DPS that don't know how to AOE... There's the newbies that are mute... There's the tanks that don't know that in this game you need to evade certain things... There's the AFK player that like to be carried.

The one that does less stupid things get my commendation, or none at all. Most of the times I give them to a good Tank, a bard that actually play something, or a DPS that didn't take unnecessary AOEs and did attack. I already saw a lot of players and did a lot of dungeons so I know when a player is doing above average. A tank that use stone skin and protection, a DPS that have swiftcast n/or resurrection, a healer that don't doze off when everyone is at full HP and actually stoneskin in the free time or DPS.