First, you can verify the scry sensor by
making an intelligence check of DC 20. You can take 20 on this.
Second, if you use the
message spell you can communicate using whispers to each other. It would take a DC 25 Listen check to hear and understand the whispers. If you cover your mouths when you do so then there will be no chance of lipreading.
Speaking in innuendo will do in a pinch. Good if your party has a decent number of bluffers. Also, speaking in an obscure language may foil a scryer's attempt to eavesdrop. My bard kept track of everyone's language and kept switching languages when speaking to an individual or a subset of the group. Since I was paranoid, I did this along with the mouth-covered
message spells that I regularly cast.
The moment you are about to do something incriminating, cast
dispel magic on the sensor. It takes an hour to recast
scry. Although a crystal ball maybe activated with a standard acton (I'm not sure, but I believe it defaults to the standard use activated rules). Same with
greater scrying.
If you are not able to dispel it, cast an illusion of a stone ceiling above your heads. Preferably an image spell that you don't have to concentrate on. Scry sensors allow you to see and hear the subject and the subject’s immediate surroundings, which is approximately 10 feet in all directions of the subject. This means that scry sensors are fixed to appear five feet or 10 feet above your head. See if you can get your DM to nail down a specific distance (especially if you are asking the question like you plan to cast it regularly). Because if the scry sensor location can't be adjusted, then using blankets and umbrellas do become a viable countermeasure.