Dry and firm is good, slimy is not. A slippery film on the cap is the clearest sign mushrooms have turned, along with dark soft patches and a sour smell. Dark marks on their own are usually just bruising from handling, and those cook exactly the same.
What to check
Lookdry caps and even color. Some darkening and opened gills are normal with age and still fine to cook.
Feelfirm and dry. Slimy or tacky means throw them out. Mushrooms are sold by weight and lose water fast, so a box that feels light for its size has been sitting.
Smellearthy is right. Sour, fishy or ammonia-like is not.
Check the boxlook under the cap edge. A veil still closed to the stem means young and mild, and it will keep. A torn veil with dark gills showing means older and stronger, so cook those this week.
Notewhite button, cremini and portabella are the same species. A cremini is a brown strain of it and a portabella is a cremini left to grow up and open its cap. You are looking at one mushroom at three ages and three prices, which is why the veil is an age test rather than a freshness test.
At home onlya paper bag beats a sealed tub, and the original punnet with perforated film is fine too. What kills them is a zip bag, which traps moisture, and moisture is what makes them slimy.
Common mistakewashing them when you get home. Wet mushrooms spoil faster, so brush or wipe them clean and rinse only just before cooking. And all of this is about cultivated mushrooms from a shop. Never judge a wild mushroom by look, smell or feel, because some deadly species look and smell perfectly good.
If they are not perfect: wrinkled and drying but not slimy is not waste, it is concentrated. Cook them today and they will taste stronger than fresh ones. If you have too many, saute and freeze them, because cooked mushrooms freeze well and raw ones do not.
Written by the team behind RIPE, a free app that reads a photo of the whole produce pile and rings the ripest ones. Every cue above is one you can check with your own hands, and it all works with no app at all.