When the detect magic spell is cast, the spellcaster detects magical radiations in a path 10 feet wide and up to 60 feet long, in the direction he is facing. The intensity of the magic can be determined (dim, faint, moderate, strong, overwhelming), and the wizard may be able to recognize if a certain type of magic (alteration, conjuration, etc.) is present. A stone wall of 1 foot or more thickness, solid metal of 1 inch thickness, or a yard or more of solid wood blocks the spell. Magical areas, multiple types of magic, or strong local magical emanations may confuse or conceal weaker radiations.
Note: This version of detect magic originally appeared in the 2nd Edition Player's Handbook for Advanced Dungeons & Dragons from TSR. Its use here is for the purposes of providing context for the campaign only.