The Collaborative International Dictionary of English v.0.48:

Perceivable \Per*ceiv"a*ble\, a.
   Capable of being perceived; perceptible. -- Per*ceiv"a*bly,
   adv.
   [1913 Webster]

WordNet (r) 3.0 (2006):

perceivable
    adj 1: capable of being perceived especially by sight or
           hearing; "perceivable through the mist"
    2: capable of being apprehended or understood [syn:
       apprehensible, intelligible, graspable, perceivable,
       understandable]

Moby Thesaurus II by Grady Ward, 1.0:

55 Moby Thesaurus words for "perceivable":
   apparent, beholdable, clear, clear as crystal, crystal-clear,
   detectable, discernible, disclosed, distinct, evident, explicit,
   exposed, exposed to view, express, hanging out, in evidence,
   in full view, in plain sight, in view, indisputable, indubitable,
   insight, manifest, naked, noticeable, observable, obvious, open,
   open to view, open-and-shut, outcropping, palpable, patent,
   perceptible, perspicuous, plain, plain as day, recognizable,
   revealed, seeable, self-evident, self-explaining, self-explanatory,
   showing, tangible, to be seen, unclouded, unconcealed, undisguised,
   unhidden, unmistakable, viewable, visible, visual, witnessable