The more you meditate on your Third eye and the more you open it, the more clearly you can see with it.
Most of the time you will see colors at first when meditating on it like Ko Johnson said, but after a while you can start seeing Spirits and Demons clearly when they are around.
It takes some time and it's easier when you enter a light trance to see spirits and Demons with your third eye, but you can really see them very clearly as if they are literally there in front of you (which they are with their astral bodies).
Just keep working on it and you will be able to see energy and such things with time.
Demons can physically interact with you and touch you or touch things in your environment with ther astral bodies. It can feel like a sunburn without pain, or simply a tingling sensation on the place of contact or some kind of pressure, like what you would feel when a person is actually holding you physically.
Since Demons have such high vibrational energies you will often feel a magnetic sensation or tingling sensation that keeps lingering for a while even after the Demon has already left on your whole body or just the place they touched you. Speaking from experience here.