This was a good question that came up this week.
If you look at a lot of art throughout the ages, and in popular media today, you see angels depicted as handsome and strong men (and sometimes women) with wings and white robes, and an occasional halo. Demons are, however, depicted as terrifying monsters, misshapen, grotesque and scary.
Yet the best you can say from scripture is that every time an angel appears face to face with humans and they are described in any way, they look just like other people, although occasionally they have brighter clothes. (see Genesis 18, Genesis 32, Joshua 5, Matthew 28, Luke 24, among others). Hebrews 13:2 even says some have entertained angels without realising it.
There is sometimes a difference when it comes to dreams and visions, and I think that given the highly symbolic nature of what often appears in dreams and visions, it is drawing a long bow to say that these descriptions are literally how angelic beings look, and not purely symbolic. (see Ezekiel 1 for a good example of this).
Demons are usually understood to be fallen angels, so it would be highly unusual to expect them to look different, even in theory.
Another thing which occurs to me is that I can’t recall any part of scripture where people see a demon. Their actions are talked about. Satan is given several highly symbolic descriptions which are quite obviously not intended to be literal.
In many ways, the idea of demons and angels being visible creatures we can see and recognise (and interact with physically) is more a cultural phenomenon than it is a Biblical one, and can be quite a distraction from following Jesus.