This is such a great question.
Personally, I fear the worst; which is that they are still continuing to bulk collect, yet are simply saying that they are not.
I'm not an expert on telecommunications.
However, my guess is that in order to detect it, we would need someone who has direct access to the access network[1] (and possibly more?).
Plus the knowledge on how to detect surveillance (sudden drop in data transfer?).
It would also require someone who's brave enough to take the risk to spill the beans.
[1] https://en.wikipedia.org/wiki/Access_network