In-Game Parser.

[Taika]

Although if you were aiming to be really good in your DPS, I'd find it hard to believe you can do so without the assistance of a parser. So I guess anyone who's trying to be the best of their class in DPS, it would be mandatory which is probably factually true.

I'd say anyone who's aiming to be good at their DD job has at least checked someone who uses parser how their DPS is. Otherwise they would have no idea.