Of what (hōn). Attraction of the accusative relative ha to the genitive case of the unexpressed antecedent toutōn.
Thou hast seen and heard (heōrakas, present perfect active indicative kai ēkousas, first aorist active indicative). This subtle change of tense is not preserved in the English. Blass properly cites the perfect heōraka in 1Co 9:1 as proof of Paul’s enduring qualification for the apostleship.