OnPageChangeListener의 onPageSelected 콜백을 이용하시면 될 듯 싶군요.
mPager.setOnPageChangeListener(mPageChangeListener)
private ViewPager.OnPageChangeListener mPageChangeListener = new OnPageChangeListener() {
@Override
public void onPageSelected(int position) {}
@Override
public void onPageScrolled(int arg0, float arg1, int arg2) {}
@Override
public void onPageScrollStateChanged(int arg0) {}
};