The circuit of off-air monitor for SSB transmitter can be built by using combination of detector product with the crystal oscillator. This circuit is used for testing the audio at one point within band. The quality of audio will still constant, if loading and tuning of SSB transmitter are same over rest of band. The LC circuit will resonated to band being used. When the transceiver is in receive mode, this circuit will spot the oscillator signal. And if the transceiver transmit the signal, the transceiver can be monitored. Here is the  schematic diagram of the circuit:

Off Air Monitor circuit schematic