You detect the warded area or squares (after 3 rounds of concentration with detect magic). Alarm is an emanation from the targeted location. An emanation is otherwise like a continuous burst from that point, affecting the area. A burst does not go around corners like a spread affect, so this may affect the area it protects depending on walls or other blockages.
Note that if there is an item or creature bearing the spell or effect that creates a warded or affected area and that object or creature is not in sight, you cannot determine the school.
Detect Magic wrote:
3rd Round: The strength and location of each aura. If the items or creatures bearing the auras are in line of sight, you can make Knowledge (arcana) skill checks to determine the school of magic involved in each.
So for alarm (which is cast on a point in space, not a creature or item), you could detect the edges and other visible areas, possibly even being able to walk around the boundaries and note the dimensions, and you could attempt to determine that it was an abjuration school or effect after 3 rounds of concentration.
If it were a darkskull, which emanates an unhallow (which is another emanation and not technically a spread, but seems to be a unique effect that permeates the 'site', so probably functions around walls or cover in the area. But either way, assume it does for this example), you would be able to detect the affected area and space of the unhallow. If the darkskull itself was out of your sight, after 3 rounds of cooncentration you could determine the strength of the aura, but you could not attempt to determine the school, per the wording of detect magic (how you would explain or describe that inability to your player is up to you.)
