There are two great features while managing variables in Nintex Workflow for O365.
When you are viewing Variables by clicking on the Variable button in the Design ribbon, you will see that the deletion button is Red or Grey. This denotes that the variable can or can't be deleted because it is in use in the workflow.
From the image above, I can delete the variables 'colApprover' and 'colApproverValue' but I should not delete 'pgApprover' because it is in use in my workflow.
The second feature is that if I click on to delete the in use 'pgApprover' variable, I will be prompted to see where the variable is in use in the workflow.
These are two quick and easy ways to manage variables in workflows for Office 365!
Note: a good way to make this even easier for yourself is to change the action's labels. Making them unique and meaningful not only help to identify the correct action to manage variables, but also to 'read' the workflow as if it was a Visio diagram.