So it's a menu item? Type C-hk and then select that item.
(Menus are implemented as keymaps, so this is just the normal describe-key
functionality.)
You can also see the non-interactive call form of the last command with C-xESCESC or C-xM-:. It's easy to figure out the command name once you have that. (thanks event_jr)
For listing all commands, most modes will list all their key bindings in their docstring, so you can use C-hm to describe the modes in use in the buffer.
As there may be commands without bindings, you could also use M-x apropos-command
to list them all (most likely specifying ^ess
as a pattern, if it uses that as a consistent name space).