You're confusing the law and the verdict with reality. drew has already said how it is possible for Ched to KNOW whether he's innocent or not. I will add that even if he thought she was capable of consent, but she was not, then he is still innocent, because he had to know she was not capable of consent in order to be convicted - that's why McDonald was acquitted - they jury decided she wasn't capable of consenting to sex with him either, but he had reason to believe she did.Ched Evans can't KNOW he's innocent as a matter of fact - his guilt or innocence depends on whether a jury of his peers believe the girl was capable of giving consent to sex. He can't know if she was or wasn't, he may believe she was on which basis he believes he's innocent, but the judgement as to whether or not she was capable or whether or not his belief was reasonable does not lie with him but with the jury and they have given their verdict.
Which is why the case is being looked at, to see if there was an error.For an appeal to be successful there has to be 'new' evidence that wasn't presented to the original jury or there has to have been an error in the procedure of the trial or actions of the judge.