From ArsTechnica:
There is an easy test to determine if a Linux or Unix system is vulnerable. To check your system, from a command line, type:
env x='() { :;}; echo vulnerable' bash -c "echo this is a test"
If the system is vulnerable, the output will be:
vulnerable
this is a test
An unaffected (or patched) system will output:
bash: warning: x: ignoring function definition attempt
bash: error importing function definition for `x'
this is a test
The fix is an update to a patched version of the Bash shell. To be safe, administrators should do a blanket update of their versions of Bash in any case.