simply check if it's null
if(prev == null)
//There is no active fragment with tag "dialog"
else
//There is an active fragment with tag "dialog" and "prev" variable holds a reference to it.
Alternatively, you could check the activity the fragment prev
is currently associated with, however, make sure you ask that after you make sure it's not null or you'll get a NullPointerException. Like this:
if(prev == null)
//There is no active fragment with tag "dialog"
else
if(prev.getActivity() != this) //additional check
//There is a fragment with tag "dialog", but it is not active (shown) which means it was found on device's back stack.
else
//There is an active fragment with tag "dialog"