Autosuggestion
Intervention by a spirit/ghost
This is where a demon or angel or some other paranormal creature somehow controls the movement of the glass or pointer. The actual mechanism here is not likely to different from that discussed for a spirit or ghost, it is just worth pointing out that if indeed you are talking to something paranormal, it is not necessarily what it is representing itself as.
If this is the mechanism a seance should be able to answer questions that a participant knows the answer to, questions that could be answered by something not constrained by physicality and questions that could only be answered by a creature with the abilities that are claimed (ie if the creature does not claim to be able to predict the future, not being able to do so does not disprove the seance's validity).
Again, for proving the validity of a seance, the creature answering should be able to correctly give an answer that could not be known or guessed by the participants.
It should be noted, however, these creatures could not be regarded as overly reliable for the purposes of testing a seance.
Firstly, if it claims to be a demon, you're not going to get good results because it should follow that a demon by nature is likely to lie. Therefore incorrect answers by a demon would not prove anything about the seance, merely indicate that demons lie. You learn little or nothing from wrong answers if self-delusion is not involved.
If it claims to be an angel, it should not lie. However, theoretically, there is nothing to stop a demon from saying it is an angel and lying its ectoplasmic head off. So a lying angel does not necessarily prove anything about the seance either.
Other paranormal creatures (what are they? - by my definition they would be spirits that were not associated with dead people) could be just about anything so you'd never know if a correct answer was just a good guess or a wrong answer a bad one, or a lie.
Of course a demon or other paranormal creature may not tell you what it is, they may well pretend to be Jack the Ripper, Aunt Beryl or a harmless little shrubbery spirit.