Is it possible to determine whether
display-fill-column-indicator-mode was enabled?
I am running Emacs 27.2.
All minor modes have an associated variable that's the same name as the function to toggle the mode. When a mode is local and has a global variant, the global variant has its own variable.
So, for example, to check whether Display-Fill-Column-Indicator mode is enabled in the current buffer, check the value of the variable
display-fill-column-indicator-mode. You don't need to worry whether it was turned on via the local or global function:
display-fill-column-indicator-mode reflects whether the mode is active in the buffer regardless of how it was activated.