The Collaborative International Dictionary of English v.0.48:

Confirmatory \Con*firm"a*to*ry\, a.
   Serving to confirm; corroborative.
   [1913 Webster]

         A fact confirmatory of the conclusion.   --I. Taylor.
   [1913 Webster]

   2. Pertaining to the rite of confirmation. --Compton.
      [1913 Webster]

WordNet (r) 3.0 (2006):

confirmatory
    adj 1: serving to support or corroborate; "collateral evidence"
           [syn: collateral, confirmative, confirming,
           confirmatory, corroborative, corroboratory,
           substantiating, substantiative, validating,
           validatory, verificatory, verifying]