There are three ways by which we can tell this(I learned it the hard way though)
When is comes to IIS7, FREB is a life saver! All we need is a FREB trace for any page which is hosted in the web site under question. A quick look at the Compact View of FREB trace will tell us what has happened with the filter.
Our good old friend Tasklist will help us here as well! Open command prompt and type “tasklist /m <name of filter dll>” and there we go! It will give us list of process which are able to load this dll successfully
This is a little longer way but works well and can come handy in situations like FREB is showing error loading filter, then ProcMon can tell why it is so!
All we have to do is
a. Install ProcMon - http://technet.microsoft.com/en-us/sysinternals/bb896645.aspx
b. start capturing data (Ctrl+E in ProcMon)
c. browse any page which is hosted under a site in question
d. stop capture(Ctrl+E once again) and search for the filter dll!
Hope this will help!