I posted a wiring diagram for my test box
here.
As you can see...if you send the switch output jack wire (lug 8) to the bottom lug of a SPDT toggle, middle lug to the output jack, and top lug to a capacitor to probe...this allows you to determine what goes to the output jack, circuit under test OR the audio probe.
I added another jack socket for the probe...grounded the sleeve and ran the other side of the cap to the tip. Then I replaced the banana plug with a mono jack plug on a spare multimeter probe. You could still use banana sockets but I had loads of mono jacks and jack sockets.
In your case you can...
Drill a jack socket(or banana post) into the side for the probe and ground the sleeve
Add a SPDT toggle near the output socket
Clip the green wire to out
Send it to SPDT lug 3
SPDT lug 2 to Output jack tip
A non polar cap from SPDT lug 1 to Probe Jack tip