It will smell bad (very musty) and have dark mold in it. The bad areas also stick together.
Moldyhay is caused by insufficient drying -- most processorsnow usedrying agents (desiccants)containingpotassium or sodium carbonateand/or preservatives (proprionicacid and acetic acid) or anhydrous ammonia,to prevent mold inthe hay by hastening curing/drying. Microbial inoculantshavealso been used to preserve hay.
Hay can also mold if kept in moist conditions, especially on a cement floor where the hay will absorb moisture.
Clover hay (heifer hay) is the most likely to mold and should not be fed to rabbits.