When they're visible, they look like ordinary people, animals, or plants, dear FP (peace to you!). This is because they cannot manifest on their own (they have no physical bodies nor "light" to reflect) and so must use a "host"... and only living things can provide host residency (so, you can dismiss any fears for the "demon-possessed" table from the swap meet - ). Because of this it is not by their appearance that they can be recognized; rather, it is by either the manifestation of their works/conduct or their self-identification.
They don't always do "bad" things; they recognize the Son... and the seed... and so will respond and identify themselves if called on to do so (by pretty much anyone), so long as the one isn't in fear. And they are not to be feared as they have no power of their own. Excluding God, and now Christ... and, in some [rare] instances, Death... the only power that a spirit (daemon)... good or bad... has is what is given it. From God, Christ... or us (humans).
One way for a bad spirit to gain access to a host is through drugs, alcohol, anger, depression, and other tools/emotions that compel the host to give his/her control "over" to the spirit. Yes, I know... there are many doctors and others who would dispute this and say that all of these things are completely biologically related and so treatable with medication. Would that that were the case, so many would find relief. Not all medical doctors/psychologists/or even scientists agree with this - some admit there are "things" that biology cannot explain. This is one of them. Don't get me wrong: medication can work - it can help the host person gain back control by helping him/her focus, fight, push through, control his/her own thoughts/feelings, etc. For some, however, medication doesn't work because the results are not the host person's desire - it is the desire of others. And for some, medication actually makes the problem worse - they lose even more control, so that when the "effects" of the medication wear off they are controlled with even more vengeance... and so pretty much have be medicated at all times. Resulting in a kind of zombie-ism.
Does "exorcism" work? You know, I had to ask that question, given the "gifts" of the spirit and my curiousity as to how they are to be used. The answer I recieved is... no. "Exorcism" is merely theatrics, designed to put fear in people and increase their dependency on the [Catholic] Church or others who use this false pretense to control people. One doesn't need "holy water" or crosses or Bibles/ancient writings or prayer cloths... or recitations of incantations, etc.... in order to cast out a bad spirit. One only needs to tell it to come out... authoratively and without fear. And if the host doesn't WANT it to remain, it must leave. Again, this is because they have no power of their own, but only what is given them. If that power is revoked, well, there you go - they must go.
I realize that this may sound... ummmm... like something from the Dark Ages. Well, okay, so be it. But I've answered you truthfully. Oppose any bad spirit and it will flee from you. Keep in mind, however, that you may not be the one "allowing place" for it... and so it may remain as a result from another's permission... whether known or unknown to them. If one is having difficulty removing a bad spirit, one might want to consider what or who the "source" of its permission to even be there is.
I hope this helps and, again, I bid you peace!
A slave of Christ,
SA