That's quite an open question.
Depends what you're doing and when you want to know when the variable was changed.
Maybe something like.....
Before a function:
Save the variable into a temp variable.
Do the function
Then check if the variable is different to the temp variable.
In a scene:
Save the variable to a temp variable in the ready function.
In the process function or a timer function check if the variable matches the temp variable, if they are different, call a signal or other function.