Fragment in focus

If you manage the fragments with a ViewPager you have two possibilities, if you work with API 24.1.0 or higher you can use the addOnPageChangeListener method to "warn" you when you change the fragment, something like this:

mViewPager.addOnPageChangeListener(new ViewPager.SimpleOnPageChangeListener() {
    @Override
    public void onPageSelected(int position) {
        //Si es el fragmento que necesitás, mostrás el mensaje
    }
});

But if you use a previous API, you can use it like this:

mViewPager.setOnPageChangeListener(new ViewPager.SimpleOnPageChangeListener() {
    @Override
    public void onPageSelected(int position) {
        //Si es el fragmento que necesitás, mostrás el mensaje
    }
});

I hope it serves you.

Good luck!