Looks, smell and feel...
Look...no obvious bugs, odd discoloration or flakes of unexpected material...
Smell... get in habit of taking a good sniff of any newly opened bag before you even look at it. You will come to recognize 'normal' and then can notice if something seems off. Moldy feed will have a musty smell or smell like a damp, mossy rock. Sometimes it can even smell sour. Basically, get used to 'normal' and be suspicious when anything is different.
Feel... is it loose and crumbly or powdery? It shouldn't be damp or clumpy. If it is then don't use it.
Any feed, from chain stores or mills, can be exposed to wet conditions. Even after you bring it home...Sometimes it is the original grain source which is the problem...but no matter what the origin of the problem the results are the same. Molding feed which can be dangerous.
Most problem bags can be detected with the 'look, smell, feel' precautions, but not always, and other contamination can go undetected until chicks are sick. So any group illness I always would be suspicious of food until proven otherwise.
So sorry
