Intellego spells are about obtaining information. The way you gain this information in-character is actually a cosmetic, secondary consideration and I would advise your troupe to approve spells that do so in any interesting, fun way - but it's up to the group to decide what spells to approve.
Information is very useful in war. You can use Intellego spells to locate your enemies, to warn you of incoming missiles, to spot an ambush, to identify a trap, to perceive weaknesses in your enemy (physical as well as magical), and so on. You could, for example, use a volley of Intellego effects to gauge the Magic Resistance of an enemy wizard to various Forms, thereby exposing which areas he is weak in and will be more susceptible to magic. Or you could use Intellego to discover things that will aid your Penetration, such as the person's astrological birth date.
Information is also very useful in social contexts. You can use Intellego to tell when someone is lying, to pick an answer from another's mind, to figure out how much money is needed to bribe him, and so on.
Let's face it, information is generally useful. You can use Intellego spells to gain any kind of information, from the location of a vein of silver in the ground to the detection of wizard's casting sigil in the magical traces left where he had cast his magic.
The way you gain this information is a bit arbitrary, and again I recommend being flexible here. You may talk to the plant, or hear a person's thought as if they were words, or see a person's emotions as if they were cog wheels in his brain, or get a foul taste of betrayal in your mouth, or have a vision of the enemy forces crossing the bridge, or so on and so forth. Be creative, make your spells get the information in fun, interesting ways.
Above all, remember that just like any roleplaying game Ars Magica works just like you, the group of players, decides you want it to.
Cheers,
Yair