There is definitely a way to tell - a simple scale reading with the pouch in it vs not in it. Regardless, I can feel the difference when the pouch is in the bag and this is actually more obvious when the NF is empty. Maybe you cannot tell, but that's not true for everyone.The pouch is not heavy at all. The strap hanging on the side of the NF weighs more than that pouch.
Put a different way:
There is zero way you’d know an empty pouch was in an empty bag upon just picking it up.