Hear me out... we know about the agro indicators, right?
Green = No Agro
Orange = Agro, but not fully on you
Red = Agro, fully on you

Meanwhile, tanks can't tell which mobs the other tanks fully have agro over and thus this will be my suggestion.
When you are a tank, you will see a 4th agro indicator: blue.
If you see mobs with the blue agro indicator, then another tank has agro on that mob and you don't need to taunt off that person. Only focus on the red indicators whilst the blue indicators are for the other tanks. Any green indicators shown mean none of the tanks have agro on that mob