They’re already fungi, so how can I tell if mushrooms have gone bad? Good question!
A slimy film is the telltale sign of a spoiled mushroom. If you catch it early and cook ’em up right away you can get another day out of your ‘shrooms, but once the slime starts, it’s a slippery slope toward the darkening in color and mushy texture that indicate mold of the inedible variety.
Remember: Whole mushrooms keep longer than presliced and stay fresh longest when refrigerated in their original packaging.