press Menu twice to get to the main options
highlight 'tracks' - enter
select 'set up' - enter
select 'data card set up' - enter
menu - select 'card info' - enter
you should know see a screen that lists various things and the space they take up
On a Quest ,as far as I know the only way to tell was answered in the second post of this thread.
There would be no point is knowing how much memory was free from the unit its self as more map data can't be added without deleting what is already installed.