Generally speaking, the more oblong an avocado, the smaller the seed is. The more egg-shaped ones tend to have a larger seed. You also want to look for avocado with relatively smooth satiny looking skin that's relatively even in color without any wrinkles. Avocados start out green, but they darken as they get ripe, so a dark color is one good indicator that it is ready to eat. The problem with relying on the color of the skin alone is that overripe avocados are also dark. The most reliable method I've found for telling when an avocado is just right is to remove the stem and look at the color of the navel. If the stem is difficult to remove and the exposed area is green, this means the avocado is not ripe. If the stem is not present, or the navel is brown, the avocado is most likely overripe. If the stem is easy to remove and the navel is a yellowish color, it should be perfect!