Where are the field names for functions in CRM?

Where are the field names for functions in CRM?

I'm admittedly a rookie at functions, but I am trying to confirm/modify field details in an existing function that's not working properly. I know there is a place in CRM where I can find the list of the system names of my fields. I thought it was under Modules>Fields adjacent to the related field labels, but I'm not seeing the list there anymore. Can anyone help? Thanks!