Detect Magic and similar spells can show the school of magic with a proper spellcraft check. Which school of magic is detected for weapons and armor with only enhancement bonuses (e.g. a longsword +1)?
Depends on the item. Here's the gist: Protection is abjuration (rings, armor, cloaks etc.); offense is evocation (weapons etc.); and other bonuses are transmutation (ability scores, skills etc.).
OpaCitiZen mentioned that this information is in the DMG 3.5 on page 213.
The Pathfinder SRD has a good listing of this kind of stuff, link is here: http://www.d20pfsrd.com/magic-items#TOC-Magic-Items-and-Detect-Magic.
It states that if no spells are given for the item, then armor and protection items show up as abjuration, weapon and offensive items show up as evocation, and items that give a bonus to ability scores and skill checks show up as transmutation.
If a spell is listed as a prerequisite for the item, it uses whatever the highest-level spell on the item is.
Hope this helps!