This command is accessible both in file management and in archive management modes.
In the archive management mode, when WinRAR shows archive contents, first, you need to select archived files and folders, which are to be tested. Then run "Test archived files" command using either Commands menu, "Test" button on the toolbar or Alt+T keyboard shortcut.
All errors found during test are displayed in the "Diagnostic messages" window. If no errors detected, WinRAR will display a corresponding message.
In the file management mode it is possible to test several archives at once. In general the operation is similar to the described in the previous paragraph, but instead of selecting files inside of archive you need to select archives and folders, where WinRAR should search for archives. When a search is completed, WinRAR will test contents of all found archives.