There is no out of the box way to distinguish between the initial load and previews of the report. Currently both the report area and parameters area are separate iframes which makes it tedious task to wire up events to the toolbar controls and thus there are none exposed. For any custom functionality, you should prepare your own UI and use the Report API
to wire it up to the report.
Still we're curious why you want to make such a distinction - can you elaborate on your exact scenario?
All the best,
the Telerik team