One possible way to display it would be changing the displayed information in the unit's status box when we are hovering over an enemy unit with the attack command active. In other words, if we have an active attack command (that is, the button has been pressed, but we haven't chosen a target yet), hovering over an enemy unit will dynamically change the information displayed in the status box to show the range of damage taking into aspect the target's defence and damage multiplier. Above that could be displayed the target's health (current and maximum), their defence, your attack type, their armour type, and the damage multiplier. The information regularly displayed in the status box would be temporarily replaced with this new information. The icon displayed would be that of the target unit's.
Naturally, we wouldn't have time to go hover over numerous enemy types in real time combat, but we'll presume such a feature would work when the game is paused, would work very well for RPG-style mods, and would allow a way to find out the damage multiplier (etc).