Tip of the day , there is actually a property for whether progress pages were sent, it's called ProgressPage.Sent. It's just a boolean.
There is also ProgressPage.Enabled, but I don't think that tells you if it was actually used.
Unfortunalty we use 7.2.....
But I found a solution in this forum... using an dummy link inserted as an object in the progresspage.
<object data="$Proxy.EnduserUrl$/progresspage/?url=$URL$" width=1 height=1><img src="$Proxy.EnduserUrl$/files/default/img/grey.png" width=1 height=1></object>
Now I can trigger for URL matches *mwg-internal*progresspage in the log Handler
In the logfile I can extract the requested file using string.replaceFirstMatch(URL,regex(.*url=),"")
We will change this after upgrading to 7.3