What would you do if you were told if one of your companions didn't exist?

[Aprophis]

There are also sellers who offer companion readings. You ask a question and they talk to your companions. Not a vessel reading or reading of a binding or anything.
I'm sure a honest seller would tell you there's nobody to contact. Also you could ask those sellers to contact a companion that you know is around with a question about other companions. Although I have read about companions hiding from the rest of the keep so they don't even know they're there, so there is that. Introverts apparently also exist in other worlds.

But as already mentioned, pendulum is unreliable. Just as with telepathy and intrusive thoughts/projection, it will also be influenced by your desires.
For training telepathy, I'd always recommend zener cards or playing cards. Something that can be easily verified but you don't know if yourself you have shuffled the deck and you can't see the card's faces. So the more cards you get right by asking your companions, the better your skill.

You could do the same or something similar with the pendulum.

But I have many things happen, including thoughtforms being sold as entities, servitors being sold as entities, entities being something completely different to what was described, empty vessels. You just learn what you can (if you can learn something) and move on.

[Magnolia]

There are also sellers who offer companion readings. You ask a question and they talk to your companions. Not a vessel reading or reading of a binding or anything.
I'm sure a honest seller would tell you there's nobody to contact. Also you could ask those sellers to contact a companion that you know is around with a question about other companions. Although I have read about companions hiding from the rest of the keep so they don't even know they're there, so there is that. Introverts apparently also exist in other worlds.

This is advisable only if you are going to ask the original seller to read their own conjurations. Otherwise, as stated numerous times on the board, having one seller read another seller's conjurations is not only unethical, it's not in keeping with the foundation of Spirit Keeping canon. Spirits have the right to privacy, they have the right to cloaking bindings to preserve their privacy which keeps any outsider from reading them.

Even in the instance you were to say an unbound spirit is being read, or that it is an old-fashioned form of spirit conjuring with no safeguards for spirit companions, you would have to have absolute confidence in the reader.