First of all you must know how the output is programmed
ie. you need the PLC program list or ladder diagram for checking
under what conditions outputs are turned on and off.
These conditions can be boolean functions between inputs and outputs, timers, counters, registers, etc...
I take a simple example....
Here is a ladder diagram
In first line "Devre 1" right you see output Q3.0
This output is switched on when inputs I0.0 and I0.1 and I0.4 are on
OR Q3.0 and I0.1 and I0.4 are on
ie. boolean function Q3.0 is (I0.0 or Q3.0) and I0.1 and I0.4
Secondly Q3.0 can never be on if I0.1 or I0.4 is not on.
So, if you want to test output Q3.0 you must connect inputs
I0.0 and I0.1 and I0.4 to logic "1", to switch Q3.0 ON.
Connecting I0.0 to logic "0" do not switch Q3.0 OFF
Only connecting I0.1 or I0.4 to logic "0", to switch Q3.0 OFF.
And so on with other outputs.....