First, be sure that tomatoes are in season. As delightful as a garden fresh tomato is, there's nothing more disappointing than the watery, vaguely red colored "tomatoes" they sell in the middle of winter. Next, give it a whiff. It should smell like a tomato, especially near the stem. Now give it a feel. It should be firm to the touch, with a bit of give under pressure. It should also feel heavy for its size. Finally, use your eyes. It should be free of blemishes or bruises. If the skin has split, the tomato needs to be consumed immediately.