Tomatoes are perhaps one of the most frequently thought of vegetables when the word heirloom is comes to mind. Heirloom plants are those that are grown from seeds that have had as little modification over the years as possible. Though minor genetic change is hard to avoid, some varieties are much the same as they were decades or even centuries ago. Some people are avid collectors of heirloom tomato varieties, and continue to seek out and grow old varieties and reconstruct their histories. Some simply wish to taste a tomato similar to that which may have been tasted two centuries ago.
Read about heirloom tomatoes before going out to buy them. Books such as "Heirloom Vegetable Gardening," by William Woys Weaver, or "Smith and Hawken: 100 Heirloom Tomatoes for the American Garden," by Carolyn J. Male are good. You don't have to read every word of these books, but taking a few minutes to familiarize yourself with several heirloom varieties will help when speaking to growers.
Browse for heirloom tomatoes from a source you trust, such as a farmers market with reputable local growers. A good way to find these is to ask around. Ask a few people who frequent local farmers markets where they get their quality heirloom varieties.
Examine the tomatoes you are thinking of purchasing. Check for any spots that look unhealthy, or where insects may have gotten to them. Only buy firm, healthy looking tomatoes. If you do find one that is going bad, you may wish to point it out to the seller.
Speak to the seller about any varieties in question. At smaller farmers markets the seller is often also the grower, and they should know a bit of history about the varieties they grow as well as how they taste and what they are good for.
Purchase a variety of heirloom tomatoes if you have never tasted any and are unsure of which you like. Instead of buying a whole bag of one kind, take home a sampler at first of perhaps four different varieties, if that many is offered where you are shopping. Taste-testing at home will give you a better idea of which ones you want to purchase in the future for your recipes.