[Mondial]

You can check for oscillation by measuring the voltage across the oscillator grid leak resistor (10) of the 6A7 tube. When oscillating, there should be a negative voltage of several volts there all across the band. If you lose the negative voltage any time while tuning, then it is dropping out of oscillation at that point.

Make sure you connect a resistor of at least 100 K or so in series with the probe connected to the coil side of resistor 10 or you will load the RF circuit and possibly stop or reduce the oscillation.