The following can be pasted and evaluated in a rails (~> 3.2) console in order to give a simple utility function to inspect the session cookie that can be taken and pasted directly from browser dev-tools.

decode_session_cookie.rb

# Add some syntactical sugar to compose Procs / lambdas like in other, more
# functional languages. Inspired by a feature request in upstream Ruby
# (https://bugs.ruby-lang.org/issues/6284).
class Proc
  def *(g)
    proc { |*args, &blk| call(g.to_proc.call(*args, &blk)) }
  end
end

# Decode the session cookie as copied straight from devtools (for ~> Rails 3.2)
def decode_session(cookie)
  uri_decode = -> str { URI.decode(str) }
  remove_signature = -> str { str.split('--').first }
  decode = -> str { Base64.decode64(str) }
  deserialize = -> str { Marshal.load(str) }
  as_indifferent = -> h { h.with_indifferent_access }

  (as_indifferent * deserialize * decode * remove_signature * uri_decode).(cookie)
end