I was browsing InstallSite today when I noticed a link to InstallShield KB Q207265:
HOWTO: Implement an InstallScript Custom Action to Detect and Display Message About Whether a Directory Exists
I'm a little suprised by this article for several reasons. The first reason is that it suggests writing InstallScript custom actions for functionality that already exists in the Windows Installer. The AppSearch pattern ( InstallShield calls it System Search ) is more then capable of resolving if a directory exists by using the DrLocator table. Also you can easily create a modal dialog with a text control that displays exists / not exists messages based on the results of the first search.
Finally the article suggests scheduling the custom action in the execute sequence with a condition of REMOVE <> "ALL" to prevent execution during an uninstall. However, this means that the message will be displayed during a silent install since the CA is in the execute sequence and there is nothing that takes UILevel into consideration.
Overall I'm really surprised by the lack of quality in this article. I know InstallShield has some extremely talented and experienced experts such as Robert Dickau so I'm not sure where this article came from.