Chris Bell has a Powershell script to find all sysadmins on a SQL Server instance:
The script below identifies the accounts on your SQL Server that have full sysadmin rights, either on their own or via an Active Directory Group.
To run this, you need a few things setup first.
A file named Instances.txt that has each instance you are going to check on its own line. Just the name, nothing more. You can see the reference to the location at the beginning of the script, just change it to wherever you put your file.
Rights to read the AD information for the domain. This way we can get the members of any groups granted access to your SQL environment.
Click through for the script.