Okay, so what I've learned: the table for Detect Evil states that a creature will detect as evil if it has enough hit dice. But familiars are also in some ways an extension of their master.
Let's say an evil wizard has a familiar, a rat for instance. Rats are not inherently evil, but I can't find the rule stating whether their alignment is modified in any way as a magic creature. I have read that they are simply neutral, but I am unable to confirm this.
Also, this may be unlikely but I feel it worth asking if the alignment of the creature's master would automatically reflect instead, as the familiar is, so to speak, a part of him.