There are a number of class markers, most associated with clothing.
They used to be hard and fast in many societies, with quite a few coming up with (usually ineffective) sumptuary laws prescribing what you could wear based on your station in life. In America they still exist, but are mostly effective at separating out the lower rather than the higher classes from the herd.
Weight is a better marker than clothes these days. Clothes can be , but only when people are TRYING to dress well. You can't usually tell social class on a dressed down person.