WiX Only handles the installation via the bootstrapper or MSI you have generated, so I would assume that it's most likely a windows installer setting of some kind when it is first created. The element ClickThrough
is supposed to be able to handle this scenario, though I don't know much about that.
The way my work colleagues dealt with this is by using IIS and an ASP.NET web service, along with a DLL that has methods to check with the web service if there is an update, and then prompts the user about the update and asks if they wish to update (did I say update enough in that sentence?).
Hope this helps.