No, there is no way to tell if the DTMF comes in-band or as feedback from caller's set. However, I thing the audio portion of out of band DTMF is not an audio feedback, but likely a short fragment of DTMF which bleeds from the switch when the switch is trying to clamp audio digit and turn it to INFO message.
For this, depending how bad (long, loud) the unwanted audio is, we can try to tune our digit detector to bypass such short digits. Please capture and send here wireshark trace with 1 call where you can see double detection, I'll see if we can work it around.
Or, you still can implement some logic in the app. Not too simple, as the audio leaks usually arrive before the out-of-band packet is sent, so, upon TDX_CST event you will need to postpone decision and wait for GCEV_CALLINFO for a reasonable time to override TDX_CST.