The general rule for combat spells is if you can see them you can affect them. If not...
However I've had a player with a blind magician so I've had to come up with rules to compensate for it. ( Oh all right, he was captured by a rather sadistic enemy who cut his eyes out. Happy? Yes I'm evil. But in my defense it was the player's fault his character got captured. I didn't railroad him. ) Anyway I came up with the following rules between the game session where the magician was captured and the one where he was rescued.
A magician needs a line of detection. In other words the player needs some method of determining the location of an opponent in a straight line from the magician.
Hearing is generally not good enough method of detection unless the character has some sort of hearing enhancement that allows the character to determine distance from sound or possibly if the character has been blind for a while and his other senses have begun to compensate. In any event a perception test is required. GM's discretion on the target number or whether to allow the test at all.
Anything that the character currently touching the target or being toughed by the target can be cast at with no problems. ( You don't get any more direct detection than that )
Detection spells can be used to determine location as long as they provide a straight line of detection to the target. Detect Life for example could be used. However only if within reason. You have to be close enough for detect life to provide a good idea on the location. "He's over that direction about 200 meters away, plus or minus 5 meters" won't cut it. Again a perception test with GM's discretion on the target number (using the spells force for number of dice) on whether the magician can "see" the taget well enough to hit him at all.
Clairovoyance doesn't help since it doesn't provide a straight line of detection. Note a Clairavoyance spell working in conjuction with an Orientation System might work, but its never come up.
Something really out of the ordinary like a Thermosense organs or being cybernetically hooked up to some sort of personal radar system could possibly work too. Though seeing through a drone breaks the direct line rule.
Some skills and spells might provide bonuses for determining if you can cast a spell at the target or not. For example the Combat Spell sense might lower the target number a little. Blind fighting skill could be helpful with opponents within melee range.
When using other senses or detection spells different things count as cover. Nearby loud noises can provide partial or full ( in which case you can't hit the target ) cover when using hearing to detect a target. Trying to hit a specific target using detect life while he's hidden in a crowd could be a waste of time. ( and if you try and fail you're likely to mana bolt the wrong target )
A character can use a perception test to ask whether or not he can detect the target well enough to target it with a spell. This counts as an action and I'll tell him what sort of target number is needed. If he needs to cast the spell right away then he'll find out the target number when he rolls the perception test. Of course at that point he's commited to casting the spell even if all he is going to hit is empty space.
Any Thoughts?