What if your hired units, as part of the NPC speech system, shout your name when attacking a hostile enemy? Together with my other suggestion "Player head above hired units", it would be another way to know if a unit is or isn't hired by a player.
- "For Xx_pastelface_xX!"
- "Death to the enemies of Xwedodah20334!"
- "Person doesn't pay me enough for this, but here I come!"
The last one is a joke.