Do a speed test using a direct connection to the Master Sockets Test connection. Cut the Bell Wire if it hasn't already been done.
Then, compare it to any other connection using your chosen filter(s).
If speeds are the same, you have a good one!
In my (limited) experience, I have never found any difference in several filters I have tried over the years.
I believe the filter has a straight through connection for ADSL- it only filters out the 'phone frequencies.
Now using BT's FTTC faceplate, so dangly filters are a thing of the past.
Allergy Advice: This post may contain traces of Meerkats.