How to detect a web update installation

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I have a demo install and use #pragma disablewebinstall to derive 2
installs, one for CD and the other for web update.

The client can now update using the demo on CD or downloaded off our website
OR from within the application by using wudate.exe

When update runs then it in effect runs the demo but detects an existing app
and switches into maintenance mode.

But how can I detect at runtime if the web update version is running?

TIA

John Fligg