What's the most reliable method of ascertaining what Tridion Hotfixes have been applied to a system (other than proper previous release documentation)?
Specifically I'd like to know for SDL Tridion 2009 but would also be interested in knowing if it's different for SDL Tridion 2011.
The most reliable method is keeping track of which hotfixes you installed manually. For example by putting every hotfix zip in a directory on the server and working from there.
The more likely one to work on every system (and not just one your setting up today) depends on a combination of file versions, manifest files (in jars), file sizes and file timestamps. Those are in descending order of preferability.