Something like untested:
if(LMIC.dataLen) { // data received in rx slot after tx
fprintf(stdout, "Received %d bytes of payload: 0x", LMIC.dataLen);
for (int i = 0; i < LMIC.dataLen; i++) {
fprintf(stdout, "%02X", LMIC.frame[LMIC.dataBeg + i]);
}
fprintf(stdout, "\n");
}