WordNet (r) 3.0 (2006):

ferret out
    v 1: search and discover through persistent investigation; "She
         ferreted out the truth" [syn: ferret out, ferret]

Moby Thesaurus II by Grady Ward, 1.0:

42 Moby Thesaurus words for "ferret out":
   ascertain, bring to light, cast about, chase, determine, dig out,
   dig up, disclose, discover, disinter, elicit, excavate, exhume,
   expose, extract, fish out, fish up, follow, grub up, hunt,
   hunt out, learn, nose out, penetrate, pierce, probe, pry out,
   pursue, quest, reveal, root out, root up, scout out, search for,
   search out, seek, spy out, trail, turn up, uncover, unearth,
   worm out