Here's an article with lots of info about null testing and below are the steps to take if you would like to perform one of your own.
- In your session, mute or delete all tracks except one track that has the TD Plus on it.
- Duplicate the track with the TD Plus on it.
- Invert the phase on the duplicated track. To do that in Cubase, you have to add the "Pre" rack in the mixer and then click on "Phase."
- Delete or bypass the TD Plus plugin on the duplicated track.
- Start playback (you should only be playing the original and the duplicated track -- all other tracks in your session should be muted or deleted.)
This will cause all frequencies that aren't being affected by the TD Plus to be silenced. So if the TD Plus is really not having any affect at all, you will hear complete silence.