When I had horses the hay bales that were on the bottom of the stack would blacken along the bottom and have fuzzy stuff growing underneath. It was almost like a film of black algae. Different molds will have different appearances. Some molds look fuzzy and pale like the kind you find on cheese that gets too old which was common at the bottom of our pelleted hay barrels.
We fed our horses baled and pelleted hay. Pellets were stored in barrels (large plastic trash cans) outside and sometimes rain would get in. The day after the rain the barrels baked in the hot sun for a while and the following morning we would find the pellets were ruined. I would hope that food stored indoors would never get that bad. Before you get a visual sign you are likely to smell it and it's usually a strong damp, musty, foul smelling odor that is obviously not the same as fresh pellets or hay.
If you have insects (roaches and ants were bad on our neighborhood) in it usually you'll notice them at the bottom after you had been feeding to your animals for a while
which introduces more mold and accelerates the spoiling process.
So anything that doesn't look or smell normal should be discarded. Anything with insects crawling of it should be discarded. It doesn't necessarily mean it's poisonous but there's a risk that toxic mold could be growing inside. Toxic mold can generate substances that can build up in the liver and kidneys and cause serious and permanent damage.
