Gently squeeze prospective avocados at the grocery store. It should be just slightly squishy. Think about an avocado like a potato. If it's not baked, it's like an unripe avocado - hard and impossible to mash. Baked, it's a little soft and mashes nicely. If the avocado is overripe, it'll feel hollow and too squishy. When you cut open an overripe avocado, it'll be gooey with brown spots inside, not the bright vibrant green you'd see from a ripe or unripe avocado. If the avocado isn't ripe yet, it'll be hard as a rock. Don't be afraid to buy avocados at various stages between unripe and ripe - once they're ready, you only have a couple days to enjoy.