In-Game Parser.

[CGMidlander]

Which brings back the issue of Dps not knowing they are doing bad because there is no way to tell. In which case, they don't try and make changes/improve because they think they're doing fine already.

There are plenty of ways to tell.

Though inaccurate for some purposes, the enmity bar can be a hint to how well a DPS class is doing -- especially if there is a gross discrepancy.

Judging from how a fight proceeds is another hint to how good or bad DPS is. (e.g. Number of Bennus; mountain buster stacks; 1st set of meteors in T9...etc.)

Closely observing if DoTs/buffs/debuffs are clipped prematurely (or if they're even being used at all) is also a way to help determine whether or not a person is playing their job correctly.

Making friends with people who do parse and asking them for data is also an option.