You want to look at the signal after the BBD and see if the BBD is correctly biased (there is signal coming through, and the signal is symmetrical, and not distorted on one side).
You will need a signal generator, that produces a sine wave or strum your guitar while measuring. With a sine wave it is easier, because the wave is symmetric and periodic. Try 0.25V at 600Hz.
Ground probe of the oscilloscope should go on ground and the other probe goes at the first BBD input. If you get signal at the input, it is working until this point. Then, check if there is signal at the output of the BBD. It will have lots of clock noise. If there is no signal adjust the bias trimmer until the signal is visible. You will have to do the same for the next stage.
The gain trimmer is used to get unity gain between stages.
You will need a signal generator, that produces a sine wave or strum your guitar while measuring. With a sine wave it is easier, because the wave is symmetric and periodic. Try 0.25V at 600Hz.
Ground probe of the oscilloscope should go on ground and the other probe goes at the first BBD input. If you get signal at the input, it is working until this point. Then, check if there is signal at the output of the BBD. It will have lots of clock noise. If there is no signal adjust the bias trimmer until the signal is visible. You will have to do the same for the next stage.
The gain trimmer is used to get unity gain between stages.