I can't do live stomach samples, either.
In my experience, however, BG which have pelleted feed as a large percentage of their diet have, shall we say, "fish by-products" that are quite different from BG which do not eat pellets.
Pelleted feed -> rather granular waste products, medium brown in color.
Natural feed -> black/very dark waste, very fine texture.
As my Grandpa used to say, "A bird in the hand can get messy at times."
Anyone else find these differences?