It is fine if you use a 3rd party program to keep track of dps. However it is still against the terms and most people don't use one. Not to mention ps4 players don't have the option. So without that little illegal tool, it is really just a best guess. I agree that you shouldn't put balance on a dps that dies 3-4 times a fight, but if a dps is using their kit properly, they should rarely stay at the #2 or #3 position on aggro list.

If you have a good way of determining how to know without use of the 3rd party, please let me know. I am genuinely curious.