I don't know if there is a specific command to do this, but the following should work:
> ps -aux | grep -ni "bash"
This will give you a print out of every bash shell running on the system. In addition to the shell, the name of the user running the shell is printed.
To figure out if it's a default, you would need to inspect the environment variables of each user - That requires root access (or so I think)
(UPDATE: Actually - The guy's answer one down is a lot better
)
Linear Mode
