Generally, squash purchased from a grocery store will already be ripe. First check the skin color - a ripe butternut squash will have a deep tan or beige color. The skin should also be firm and free from any soft spots or blemishes. Additionally, the stem of the squash should be dry and corky. If you gently tap on the squash, it should sound hollow, indicating that it is fully mature. Lastly, a ripe butternut squash will feel heavy for its size.