<p>What's the best way in c# to determine is a given QFE/patch has been installed?</p>

The most reliable way is to determine which files are impacted by the QFE and use
```
System.Diagnostics.FileVersionInfo.GetVersionInfo(path)
```
on each file and compare the version numbers.
edit: I think there's a way to check the uninstall information in the registry as well, but if the QFE ever becomes part of a Service Pack or rollup package that might report false negatives