Most of the time I see them as I would see any person, but only for a few seconds that clearly (2-10 seconds). Then I will continue to see them, however they are like finely detailed vapor but with faint color that may "flicker" back to being perfectly clear again. I can see them when I look directly at them, but I can also ignore them and look through them when they're not flickering.
I've been able to see them this way since I was a child. My family on both my mother and father's side have all had times they saw spirits crystal clear. But I wasn't raised that it was my imagination because my family all had their own experiences, so I never lost the gift.
My friends would always ask me to stay over at their house to see if I could see what they had on occasion. But they wouldn't tell me what they saw or where. I'd walk around and describe what I saw to them and they would always get the wide eyed "Yes! Oh my god! You saw it too!" It made them feel better to know they weren't crazy and heck, it made me feel less crazy too. =p
I wouldn't want to see them any other way, because if they don't look like detailed vapor or flicker, but remain consistently crystal clear, I know they are manifesting and then I get on guard very quick! The only time a spirit can physically harm you is in the short time it manifests (and very few ever get that far unless someone stupid gives them the energy to). But if that does happen, magick is still your best friend. This is why I push that people should wear physical amulets and put physical wards around their home.
When I sense one in the room, I use my mind's eye (remote viewing) to see them instead of turning around and looking at them. I don't like making a big deal out of it and won't show signs I know they are there unless they directly try to communicate or enter my personal space. It prevents the ackward conversation that usually starts up with a restless spirit asking "help me?" Or the spirit slowly walks toward me until they're inches away. *sigh* I don't talk to every stranger on the street, especially the crazies for this very same reason, and I don't make a habit of speaking to every spirit that I cross paths with. If they let me be, I let them be.