I usually get a 10lb bag of onions which lasts me about 2 months +/-.
Some may get a little soft as time goes on ( and who knows how ling they have been sitting in the market.).
Most common things I'll notice is soft, slight discoloration (usually darker) and maybe. align sliminess to it.
If its just the outermost layer, I just remove it, rinse it, and use it .
If its affecting a few layers, I usually just chuck it.
If it smells bad or like its rotting, or any evidence of flies, out it goes no matter how many of few layers are affected.