Of course you can't determine whether something exists or not. There might yet be other probabilities out there that actually are objective. The fact that we have not discovered any such thing, however, is telling. Absence of evidence is evidence of absence. Therefore, it is likely--not certain, but likely--that no such probabilities exist. If your claim is that we cannot be certain of this, then of course you are correct. Such a claim, however, is trivial.