When I was studying Buddhist meditation, the question came up of 'mystical experience' while meditating.
It was mostly dismissed as tricks of the mind. Things to be ignored. The final advice came, that if it is something other than your mind playing tricks, it will make itself known ... still a tricky one, much like Dick's yardstick.
Yes... what does this mean in the context of the question whether angels and demons exist?
On one hand, in the context of Buddhist meditation, aimed at realizing "ultimate Truth", any specific content of the field of experience
, be it an itch on the nose or a vision of lights and faces, heavenly beings or demons, deep thoughts on the meaning of this or that, must be considered "not it".
Depending on the meditation method in use, such experiences of "content" can be observed or examined for their tell-tale characteristics: impermanence, emptiness (of any essence), and "suffering" (dukkha). Such "content" can be physically "real" - the wind in one's face or the cramp in one's buttocks-, or if not physically real, they can still be causal (such as worrying thoughts causing the heart to beat faster) - and it all doesn't matter in the context of such a meditation, it is all simply scenery, which can be entertaining for a while (or not, depending on the meditation school) but should not lead the meditator astray from the task at hand (as defined by the meditation teacher).
On the other hand, in the context of our everyday lives, the Philipp K. Dick yardstick is quite useful, I think, given the qualifications I mentioned above. Further, this is the context of deciding the reality of angels and demons, which if present are very much part of the "content" of our lived experience, and decidedly not "ultimate reality".
All of the above is my opinion, reflecting what I have learned about Buddhist meditation from my teachers and my practice, but without any claim to being the only or even just a correct way of meditating.