How can I see how much spam is blocked by specific junk filters I set up?

I've added several entries to my Blocked Senders list, but I'd like to know many messages each one is blocking. Entries that do nothing or almost nothing, I'd like to get rid of. Also, by knowing how well each is working, I might get ideas for more general filters, e.g., to block an entire domain instead of a bunch of users from that domain.

So how can I see how effective each of my Blocked Senders filters is?