I do not personally know anyone who has physically seen demons (I know OF people who have, and I could name a few), but I can sense demons as if they were physically there (basically, I can sense them as well as if I was physically seeing them, without actually seeing them). I also know my friend has seen demons in his mind, and he thought nothing of them until he saw a drawing made by some satanist and he could recognize the demons in the drawing.
Hi Zenocrates. Welcome! According to the Biblical account, demons are spirits who have the need and desire to dwell in living things, like animals and people. So if you're looking for demons, perse, you won't find them. When you look at certain weird acting animals or people, you my, I say may be looking at a living body in which a supernatural entity dwells.
If you look at any given
true Holy Spirit born Christian you are also looking at a living body in which a supernatural entity dwells according to the Bible. See I Corinthians 3:16 and especially I Corinthians 6:19 "Know you not that your bodies are the temples of God........"
Also according to the Bible, you may be looking at what you believe to be a person and may be, in fact, looking at a supernatural angel from God. This kind of thing happened to a number of people in the OT and angels appeared Elizabeth and Mary in the NT, but if I recall correctly, I believe they didn't look like humans. Possibly because having the indwelling Holy Spirit and the Biblical cannon, there is not the need for angelic messengers from God. Jesus appeared to folks after his resurrection as a supernatural entity, moving through walls and obfuscating his appearance on occasions.