How do I check many RAR files for corruption at once?
Using 7-Zip, you can select multiple RAR or other archives and select Test in the 7-Zip File Manager to test the integrity of all of them.
Alternatively, you can enable 7-Zip's shell integration feature1, use Windows Search to find all of the RAR archives (in Windows 7, use the search string type:rar), then select all of the results of the search, right-click on the results, and under 7-Zip, select Test archive.
1To enable shell integration, open the 7-Zip File Manager, select Options from the Tools menu, select the 7-Zip tab, and check the Integrate 7-Zip to shell context menu check box. Note that this requires administrator privileges; in Windows Vista and Windows 7, right click on the icon for the 7-Zip File Manager, click on Run as administrator, and click through the UAC prompt.