WordNet sense 1 (your basis for belief or disbelief; knowledge on which to base belief):

grounds

WordNet sense 2 (provide evidence for):

prove, show, testify, bear witness

WordNet sense 3 (give evidence):

tell

WordNet sense 4 (provide evidence for; stand as proof of; show by one's behavior, attitude, or external attributes):

attest, certify, demonstrate, manifest