Testing complex filters can be time consuming and there can be a lot of trial and error work, especially when regular expressions are involved.
It would be useful to be able to run a filter against that one pesky message that you can't get to match, and see what's going on inside. Right clicking a message could show a "Test Filter" option. Selecting it would display a list of available filters.
Selecting a filter would run the filter against the message and display the results. This would include each rule in the filter, whether it was matched or not, and the result for the filter as a whole.
For RegEx rules, showing the text that was matched would be useful, for those situations where the rule is too greedy (matching too much).