I came down on the "NO" side in that discussion, using the reasoning that you cannot target what you cannot see. (I.e. if someone is invisible, you can't target them with magic missiles, even if you know exactly which square they are standing in.)
However, if you were able to discern the dominate spell seperately from the creature and any other spells it has on it, then I would allow you to target it.
Basically I would require you to use a Detect Magic and three rounds of study (and successfully identifying the spell), or a similar effect before I would allow you to target a single spell that has no visible effects.
If the spell is visually obvious (Fire Shield, for example) then I would allow you to target it directly.
This is how I rule it, based on my understanding of the spell, and the rules for targetting things with spells.
However, if you were able to discern the dominate spell seperately from the creature and any other spells it has on it, then I would allow you to target it.
Basically I would require you to use a Detect Magic and three rounds of study (and successfully identifying the spell), or a similar effect before I would allow you to target a single spell that has no visible effects.
If the spell is visually obvious (Fire Shield, for example) then I would allow you to target it directly.
This is how I rule it, based on my understanding of the spell, and the rules for targetting things with spells.