The spent flower should drop off naturally. If there is a "bean" left behind, that is the seed pod and it needs to dry naturally on the plant in order to harvest viable seed. This info is courtesy of Seachanger on the Dave's Garden forum:

The stem holding the pod begins to wither and go yellow. The pod itself may change color to a lighter green, but not always.
When you feel the pod it is much softer.
Ripening time from 4 to 9 months, being a spring pod it should be quick.

Brugmansia's rarely grow true from seed due to cross-pollination, but you might end up with a unique hybrid depending on what varieties are being crossed. Hope this helps! =)

Thank you!

I actually do deadhead mine as I don't care to grow from seed, so I do not need the seed pod. I just break the bloom off at it's base so as to not have ugly drying blooms on my brugs. I think it is just a matter of opinion and just do whatever works best for you.

Mine was covered in spent blooms and when I looked across the back yard, it looked like a bunch of old rags hanging in the air. I had to deadhead.