Q:

How to know if a user has activated the "bigger font" accessibility option in android?

How can I determine if a user has the "bigger font" option activated in the accessibility page of an android device?

A:

public float fontScale
  Current user preference for the scaling factor for fonts, relative to
  the base density scaling.

You can get current scale:
float scale = getResources().getConfiguration().fontScale;

Default value must be 1. You can debug and see the value for "bigger font"'s value.
http://developer.android.com/reference/android/content/res/Configuration.html#fontScale

