Separately, for all failed trades, the keypairs commitments are verified and any violations detected are broadcast to the network. (Or all the peers can monitor all the other trades)
Violations of cut and choose mean that the party doesn't publish the keys. That means you can't prove it.
What you need to do is show that the person agreed to pay the fee and then refused to build the transaction.
The proof would be the signed fee transaction but missing the other party's signature. The accused party could then respond by publishing the full transaction and "clear their name".
If fraud was successful, then you could publish the locked output and show that the published private key didn't match the committed public key.