If the crop protrudes past the abdomen when not completely full or for long periods of time, or looks saggy at all, the crop is too big. If you think your bird's crop is too big I would bring to the vet, an over-sized crop can be caused from yeast and other infections (my budgie's was caused from yeast as well as me overfeeding him when he was still being handed). Certain medications can cause crops to sag too
I wouldn't say there are any warning signs or things to avoid (Other than the stuff that comes with yeast or other ailments that can cause an over-sized crop) An oversized crop by itself is a condition, not an illness, and it's rare for it to develop on its own (again, yeast can cause it) in adult birds. Usually, it happens to babies while their crop is still growing and they get overfed