How could I list all super users?

If you just need to list the sudoers listed in the sudo group, I think that the best way to do it would be to run this command (which should be computationally lighter than any of the other commands in this answer):

grep -Po '^sudo.+:\K.*$' /etc/group

Also as suggested in the comments by muru, the format of the entries in /etc/group can be easily handled by cut:

grep '^sudo:.*$' /etc/group | cut -d: -f4

Also again as suggested in the comments by muru, one can use getent in place of grep:

getent group sudo | cut -d: -f4

Any of these commands will print all the users listed in the sudo group in /etc/group (if any).

Command #1 breakdown:

• grep: Prints all the lines matching a regex in a file
• -P: makes grep match Perl-style regexes
• o: makes grep print only the matched string
• '^sudo.+:\K.*$': makes grep match the regex between the quotes

Regex #1 breakdown:

• Any character or group of characters not listed matches the character or the group of characters itself
• ^: start of line
• .+: one or more characters
• \K: discard the previous match
• .*: zero or more characters
• $: end of line

Command #2 breakdown:

• grep: Prints all the lines matching a regex in a file
• '^sudo.+:\K.*$': makes grep match the regex between the quotes
• cut: Prints only a specified section of each line in a file
• -d:: makes cut interpret : as a field delimiter
• -f4: makes cut print only the fourth field

Regex #2 breakdown:

• Any character or group of characters not listed matches the character or the group of characters itself
• ^: start of line
• .*: zero or more characters
• $: end of line

As it stated here I consider the simpliest way to discover with -l & -U options together, just type users it will list e.g.: John then:

If the user has sudo access, it will print the level of sudo access for that particular user:

  sudo -l -U John
  User John may run the following commands on this host:
     (ALL : ALL) ALL

If the user don't have sudo access, it will print that a user is not allowed to run sudo on localhost:

   sudo -l -U John
   User John is not allowed to run sudo on localhost.

As it has already been stated, the answer can be found on Unix & Linux Stack Exchange:

This shows that user "saml" is a member of the wheel group.

$ getent group wheel
wheel:x:10:saml

The only difference is that the group in Ubuntu is not wheel, but sudo (or admin in older versions of Ubuntu). So the command becomes:

getent group sudo