How to know when fragment actually visible in viewpager

Of course. Assuming that viewPager is your instance of the ViewPager, use: viewPager.getCurrentItem().

Within your Fragment you can check if its instance is visible to the user like so:

@Override
public void setUserVisibleHint(boolean visible) {
    super.setUserVisibleHint(visible);
    if (visible) {
        Log.i("Tag", "Reload fragment");
    }
}

Always make sure that you search for answers throughly before asking your question. For instance, the first place you should check would be: https://developer.android.com/reference/android/support/v4/view/ViewPager.html

as per @Matt's answer setUserVisibleHint is deprecated

so here is alternative way for this.

    @Override
    public void setMenuVisibility(boolean isvisible) {
        super.setMenuVisibility(isvisible);
        if (isvisible){
            Log.d("Viewpager", "fragment is visible ");
        }else {
            Log.d("Viewpager", "fragment is not visible ");
        }
    }