If all RadWindows on the page are opened through a RadWindowManager you can use the GetRadWindowManager().getActiveWindow() method to see if there is an active window (i.e. a shown one).
You can try it out with the following simple code:
one = radopen();
two = radopen();
three = radopen();
When it alerts null then there is no RadWindow opened. Comment out some of the close() calls and you will get the RadWindow object.
Note that this is a client-side method and such a check cannot be performed on the server. It can be done before a request is initiated and a flag in a hidden field can be raised to transfer the information to the code-behind.
the Telerik team
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