There goes the Inazuma(電) has a folded ponytail(乚) method of identifying...
Semi-relatedly, in one of the child pics, Ikazuchi's hair demonstrates that even if you put it in a folded ponytail, it's gonna do the Ikazuchi thing. :)
It's my understanding that they have different hair colors. Inazuma's is typically a lighter, more defined brown while Ikazuchi kind of leans towards reddish brown.