Here's how I did it:
Disconnect the vacuum line going to the timing advance.
Hook that line to your vacuum gauge.
See what the gauge reads.
Then start disconnecting and plugging vacuum lines.
While watching the gauge, if you disconnect a vacuum line the needle will fall (because you just created a huge leak). Plug the line you just disconnected (golf tees work well).
After you plug the leak, does the gauge return the same position or do you now have more vacuum?
If the vacuum is unchanged, than there is no leak. If the vacuum increases, then you have a leak in part of the system that is disconnected.
Someone may have a better way. That is just the way I find leaks.
A more traditional way is to leave everything connected and spray vacuum lines with carb cleaner. If there is a leak, the carb cleaner will get sucked in and make your idle jump for a second. That also tells you exactly where the leak is.
C3's are infamous for vacuum leaks thanks to our headlights.
Hook the vacuum guage to the vacuum line going to the headlight actuator and check the vacuum level, then do the other one. If the readings are the same at the vacuum line, then the actuator has a leak in the diaphragm. If the readings are different, then the leak is in the line between the two actuators.