https://github.com/gelstudios/gitfiti and similar are pretty easy to spot. As a user I'd be biased to someone who used that for fun over someone who ('faking' or in all seriousness) has a ton of repos with lots of activity that are ultimately just things like node's left-pad...
Checking their actual commits. If their activity is menial like adding a letter to a file over and over, they obviously aren't legitimately writing that much code.
