Do you have a threat meter? I have just been guessing based on that for my WWWWPa

There is an addon called Magic Targets which Google tells me works in Classic. I tried it out in retail a long time ago but ended up not using it.
https://www.curseforge.com/wow/addons/magic-targets

It doesn't do threat, but it lists the mobs and who are attacking them and who they are attacking. Something you could try out.