If you use Chrome https://github.com/gwwar/z-context is a simple extension to see:
- If the current element creates a stacking context, and why
- What its parent stacking context is
- The z-index value
and important, like aprohl5 said: The z-index property can affect the stack order only if the position is explicitly set to fixed, absolute, or relative.
This is a nice way to mantain order with Sass https://www.smashingmagazine.com/2014/06/sassy-z-index-management-for-complex-layouts/