Would a truth spell work on a demon or can you rely on your intuition? I know that with intuition, the stronger it is, the more chance of knowing who's lying and who's not, but would such intuition work when it comes to demons? The reason why I ask this is because I want to be able to make demon friends, but I tend to be too trusting at times and I don't want to be easy prey. All help is appreciated.
Not all demons are evil, nor are all demons "predators" looking to hurt people. Intuition can only get you so far. If you do not know how to ward, or if you do not have wards or enough experience to see past a glamour or a lie, you shouldn't be working with spirits.
Now, a truth spell could work. However spirits, if they wish to trick you, have ways around it. Glamours to mask their true energy and self, riddles they could form that would be telling the truth, just in tongue, or even just full on impersonation of spirits you do trust.
The best way to protect yourself from the spirits who actively wish to hurt you? Ward. Wardward and ward again. You can set up wards to prevent entities with bad intentions from coming through, and you can program a ward to deflect any entity that wishes to lie to you.
If you cannot ward, and don't wish to listen to my warnings, intuition can help with some things. If a spirit tells you "I can help you unlock your true power!" or something along those lines, run away and don't look back. Using common sense can help, it doesn't even need to be intuition. A manipulative or abusive spirit will often play tricks and try to make it seem like you need them. A spirit who does wish for companionship will not, but any spirit can seem to "turn" on even the best of spirit workers.
I work with demons and demonics almost daily, so you can PM me if you'd like, and I can answer more questions.