I primarily look for signs of the ability and interest in self-teaching. A front-end developer exploring the backend? Great! A backend developer playing with probabilistic systems? Super!. A java developer branching out into Scala/Clojure/Rust/Haskell etc? Even better!

For those that won't look at Github because not everyone has an account. That doesn't make sense. Whether or not the candidate has a Github account is not a litmus test, its just additional useful information. If they have an account that may tell you something useful about the candidate. If they don't have an account, then you have to rely on other approaches to get that information.