I think in this case, canon is not quite relevent I am affraid to say, like it or not. We are lacking an armoury of divniations which are not feasible in nwn for whatever reason and thus bieng by removing spell visual effects, we lose even more worth of the nwn version of the divination, therefore I proposed this in IrC.
Count the number of Orisons/Cantrips a Druid/Cleric/Wizard/Sorc has...
Allow the use of "Detect Magic" to be 1/orison. This limits the ability to the ammount of cantrips/orisions the caster has.
Detect magic: An extending sphere around the caster draws forth the powers of magic, allowing the caster to centre his concentration on the direction of the strenght of magical fields.
This means.
Like ranger tracking, the caster detects from his point, the Strenght and Direction (ONLY) of magical fields near him. This could be players or objects marked with a relevent magical strenght. With players, it checks to see if the player has any buffs, and counts them. Not sure How this can be done but I'm sure it can. Less buffs = less strenght. Someone with one buff is not detected.
For instance.
Four Spellcasters standing around the detecting cleric. One is invisible only. One is wearing 4 "buffs". One is carrying a magic sword out and has four buffs on one of which is invisibility. One is a spellguard glowing like a christmas tree.
The cleric use detect magic.
The invisible spellcaster only has one buff and is considered too weak to be detected in the background magic of the underdark.
The spellcaster wish four buffs on is detected as a medium strenght "signal" and his direction is given, this case it is North. "You feel the pull of something a bit more than weak magic from the North"
The spellcaster with the sword is detected. His sword counts as two aditional "buffs" (+1 ab +1 dmg). You feel a strong pull of magic from the south east. Note, no distance, only direction.
The Spellguard is not detected due to spellguard robes.
Additionally, the spellcaster detects a strong magical field from the west. This happens to be a door with a powerful magical lock, and the door is marked "magical field_ strong". The spellcaster doesnt know What it is, if it is an object or player, just that it existst.
"OMg That is teh Poweful".
Actually no it's not. Only used in the right places will it be useful, else you just get a hundred false readings.
What also limits it is you only get the ability per each orison, as if you are casting the spell from memorised. This is stored from each use as a variable, which eventually renders cantrips useless that are memorised. See Charm Person, by me for how that works.
Yes you all cry, this breaks the pnp rules. So what, alot of spells do. But it can be worked around. You can always throw in a spell that counters this, and marks the caster/wearer undetectable.
Personally I think this is a very good compromise and remember, cantrips cap at about, 6 I think, or 5 anyway.
Additionally, diviners can get bonus uses, so SF Divination gives +1 use, GSF gives another +1 (+2 total).