I'm a Wintel systems & infra person rather than a programmer, so maybe it's different. But yeah, generally, I can tell from the way the thing is written, from the depth of previous experience, from the types of companies the person has worked at, from the career progression.

It also helps that I live in Melbourne, Australia - the IT world here isn't massive, and if you haven't worked with the person the odds are you know someone who has. :-)