The Collaborative International Dictionary of English v.0.48:

Guidepost \Guide"post`\, n.
   A post at the fork of a road, with a guideboard on it, to
   direct travelers.
   [1913 Webster]

WordNet (r) 3.0 (2006):

guidepost
    n 1: a rule or principle that provides guidance to appropriate
         behavior [syn: guidepost, guideline, rule of thumb]
    2: a post bearing a sign that gives directions or shows the way
       [syn: signpost, guidepost]

Moby Thesaurus II by Grady Ward, 1.0:

21 Moby Thesaurus words for "guidepost":
   arrow, blaze, compass needle, direction, direction post,
   finger post, fist, guide, guideboard, hand, hour hand, index,
   index finger, lead, lubber line, milepost, minute hand, needle,
   pointer, signboard, signpost