Look and smell first. Fresh spinach should be bright to deep green and have a neutral, leafy scent. Discard any leaves that are slimy, mushy, or emit a sour, off odor.
Common spoilage indicators:
If only a few leaves are bad, remove them and use the remaining greens immediately. Cooked spinach that has developed an off smell or sliminess should be discarded.
When in doubt, throw it out. Consuming spoiled leafy greens can cause stomach upset. To reduce waste, plan to use fragile greens earlier in the week and store properly to extend life.