But that does not answer the question of how do we know whether he committed tax fraud or election fraud?

We don't know - the verdict sheets are at the link below, and they don't provide that information.

To clarify one point - to find him guilty of the crime of falsifying business records, the jury doesn't need to find that Trump committed tax fraud or election fraud. The statute simply requires that the false business records be intended to conceal a crime, not that the crime actually be committed or that the defendant be the one to have committed the crime.

Example: I am a security officer at my company's building, and I make an extra key card for an accomplice of mine so that he can come in and rob the building. I falsify the key card records to hide my tracks. The accomplice gets cold feet and never does anything. I'm still guilty of felony falsifying business records, because the purpose behind my making the false business records was to conceal a crime. The fact that the crime never occurred, and wasn't to be carried out by me personally, does not matter.

thehill.com - Trump hush money trial verdict sheet