Although @dunedin15's fantastic answer has served me well on a number of occasions, it can give inaccurate results for some edge-cases, such as when debugging build settings of a static lib for an Archive build.
As an alternative, a Run Script Build Phase can be easily added to any target to “Log Build Settings” when it's built:
To add, (with the target in question selected) under the Build Phases tab-section click the little ? button a dozen-or-so pixels up-left-ward from the Target Dependencies section, and set the shell to /bin/bash and the command to export. You'll also probably want to drag the phase upwards so that it happens just after Target Dependencies and before Copy Headers or Compile Sources. Renaming the phase from “Run Script” to “Log Build Settings” isn't a bad idea.
The result is this incredibly helpful listing of current environment variables used for building:
