You can tell a tomato is ripe when the flesh feels soft and gives slightly when you press your thumb into the side of it. Avoid hard tomatoes or those that aren't red yet. Good tomatoes can be found at farmers' markets, roadside stands, and in your own backyard if you have a veggie garden. In the grocery store look for "heirloom" tomatoes. Do not store tomatoes in the refrigerator unless they are so ripe and soft that they are about to rot.