If you see someone dealing no dmg then you can just look in-game what they are doing.

Caster are especially easy to figure out why they deal so much less dmg.
Give them advice in what they could improve.
If they continue to play like a bot or even get offensive then you have a reason to yeet them out of your group.

You don't need a 3rd party tool to proof that someone is not playing correctly. But it helps to faster identify the reason to why everything dies so slow.