When choosing an avocado at the grocery store, look for ones with firm, smooth skin (no weird hollow spots, squishy spots, or bruises). You can check to see if an avocado is past its prime by removing the small stem - if it's still green underneath, your avocado is still okay. If it has turned brown under the stem, the avocado is past being ripe - don't purchase these. Let the avocados ripen on your counter for a few days if they're particularly firm.