If you need to know if a host is in maintenance mode via the command line, SSH into your server and run the following:
vim-cmd hostsvc/hostsummary | grep -i maintenance
This will return the following line (in this example the host is NOT in maintenance mode):
inMaintenanceMode = false,
To see the entire host summary printout without filtering everything apart from maintenance, run:
…but you’ll soon see why grep is useful here!