This guide applies to the Chicago Region and is not complete for other regions. Disclaimer

Key to Mushroom Genera with Free Gills.
Step 2: Volva Presence

Universal veil present, forming cup (volva) at stem base, look carefully. Growing on trees, woody debris, soil, or on other mushrooms.
Go to Volvariella.

Universal veil absent. Growing on wood, wood chips, occasionally on ground from buried wood.
Go to Pluteus.

All images by Patrick Leacock unless noted.