In this case, Z26-13 is 'held' Hi by the R20 3k resistor upto +5v.
The Capacitor C30 is there for noise filtering, removing some spikes from the switch contacts etc.
So, the slam switch (or output from the mod) on (connector J5-10) will need to go to 0v to change the state of Z26.
Assuming you know that Z26 is in 'invertor' whatever appears on the input (pin13) will be the exact opposite on the output (pin12).
Other than that, you would only know what Hi/Lo signals to expect when you know what the pins do on the chip.
You'd need datasheets on the chips to learn which pins need to be Hi or Lo to do their job etc....
MM.
