Although I have superb internet with a moderate to high end gaming rig, I even exhibit issues once in a blue moon when my region is having outages for instance. However the difference between me and some of my current players is that I will make everyone aware that I am having issues, usually at least an hour before raid time unless it is something that happens mid raid, and I will either cancel the raid or find a reasonable replacement for myself. I will not try to play with over 500ms latency, miss easy mechanics, and drag down the raid with me. I have however witnessed players who will. Or as other posters have stated I have seen players who claim that their inability to press the one or two button in a timely maner was in some way related to latency. As with mods such as DBM which you can check to make sure everyone in the raid is not only running it but has the most updated version I am simply looking for a way to monitor latency by people in the group to be able to diagnose and respond to any of these issues.
As previously stated. There does not exist a solution for this in-game currently, and if it were it could easily be faked. Fire up 10/25 command prompts on a seperate monitor, get your raiders hostname/IP and run an individual ping -T on each of them.