Generally

Generally you can't tell much about a person by the way they look. I've met someone who looks like a very hippyish young Jim Morrison (from The Doors) who's a member of the BNP (British National Party) which is related to the National Front. I also have met someone who dresses in sports wear all the time and goes out with a goth woman.

I would be very worried by hate tattoos and swastikas but not by a lot of piercings and tattoos because I've met people with quite a few of those who are really pleasant people to be around.