Every door is a little different, even the doors created by the fae. Some are permanent; some move; some are transient; some will take you to different places depending on the day of the week or the phase of the moon; some will take you back in time; some will take you forward in time; and some... well, a magical door can pretty much take you anywhere. There are people, normally demons, who know what each door will do, but none of this information is written down, even though most of the doors cannot be used by the walkers.
The permanent doors created by the fae have beautiful stone frames around them, as a symbol of their existence. Each is guarded by at least one gargoyle, although these gargoyles aren't the same as the ones you'd see on Earth. All of the gargoyles have a stone shape and a human shape. It is due to the gargoyles that demons can have trouble travelling through the doors the walkers use, but there are some who are happy to let a demon use one of their doors.
None of the other doors have a physical symbol. They are purely magical, so if you don't know they're there and you don't have the ability to walk through them without a tattoo then they'll have no affect on you. Often walkers come across these doors but have no idea. It may be possible to create tattoos for these doors, if someone with the right magical ability knew they existed, but it would be a difficult thing to do.