If you have a scope this will be more straightforward. You put a signal through your circuit, measure the peak voltage, then put a load on it and see how it reacts. The peak voltage will start to go down with some sufficiently low enough load. It might be 100k, 10k, 1k, 100, 10, hard to know in advance (as I'm not familiar with your circuit).

Then you can get the source impedance using this formula, which I stole from myself.

Source impedance (ohm) = (Rload) * (Vopen - Vload)/(Vload).

Example:

Suppose Vopen = 3 volts.

Rload = 1k

Vload = 1 volt.

Rsource = (1k) * (3 - 1)/1 = 2k ohms.