In case of binary load, the variables must be created in the Variable Overview menu. In case of normal load, the variable could be created in either the Variable Overview manually (so there is no SET nor LET in the script or hidden script) or in the script or hidden script, of course.
If you mean where the values where assigned to the variables, there are several places: script, hidden script, Variable Overview, Triggers in the Settings menu, Document Properties, Triggers (all of those can have an action External, "Set Variable") and in the module editor (Ctrl + M) via macro.
Hope that makes sense.
I am afraid, you CANNOT check AT ALL to see where the previous developer might have SET these variables!
Try this scenario:
1) Add some variable into script, say LET A=100.
2) Run the script (Ctrl+R).
3) Change "LET A=100" statement into "LET B=200" inside your script.
4) Run the script.
And now you can see in menu "Settings" - "Variable Overviw" both of them,
that is A variable and B variable, too. Is it strange? For me it was very strange.
So, QlikView remembers all your SETTINGS/LETTINGS of variables inside your script
althoug you have deleted them inside this script for a long tme ago.
First I was very surprised by this behaviour of QlikView, but when you will realise that you can define variables not only in script but also in some dialogues, then - I think - we cannot classify this behaviour of QlikView like ERROR, but like FEATURE.