Autosuggestion
Intervention by a spirit/ghost
Self delusion is where the belief of the participants drives them to subconsciously control the movement of the glass or pointer. A sort of not quite intended fraud.
This is very like autosuggestion except that one or more of the participants actually controls the movement of the glass, albeit subconsciously. They hear the question, they know what needs to be spelt out and their fingers subconsciously push or pull the glass in the direction of the required letter. This is interpreted by the other participants as discussed in the Autosuggestion Section and you get real words spelt out.
If this is the mechanism you could find out by asking a range of questions that you know that will be answered in a particular way by one and one only participant. If you know that your sister's birthday is 26 January but Geoff always thinks it's the 25th, then ask it. If the spirit says the 25th, then you start to suspect that Geoff is controlling the glass. Try then to focus on questions that would reveal that Geoff is controlling the glass. This may be complicated if more than one person is controlling the glass at different times.
Alternatively you could ask questions the answers to which answers would reveal the ignorance or prejudices of the participants. This would be useful for when the "spirit" is supposed to be a historical figure. If the details were wrong but accorded with what the participants already thought then there is something fishy going on.
(This discussion relies in part on thoughts provided by Mike Condouris. Errors and misrepresentations are mine.)