The user-data for an instance is available for any process on the instance to retrieve at this location:
http://169.254.169.254/latest/user-data
The DNS name "instance-data" resolves to that IP address, so if you trust DNS to be up, you can also use the easier to remember:
http://instance-data/latest/user-data
Here are the Amazon docs:
http://docs.aws.amazon.com/AWSEC2/latest/UserGuide/AESDG-chapter-instancedata.html