You can tell when a tomato is ripe by its color, texture, and smell. A ripe tomato will have a deep, even color, and will be slightly soft to the touch. It should also give off a slightly sweet, earthy scent. If the tomato is still firm and has a greenish tint, it is not yet ripe. If it is too soft, mushy, or has brown spots, it may be overripe and no longer good for eating.