There are 2 methods for making homemade tomato juice. One method takes a lot more time and involves cooking the tomatoes, the other is super simple - juicing the raw tomatoes! Since cooking kills some nutrients I almost always prefer juicing raw fruits and veggies for making juice.
First, check the outer appearance of the tomato. It should be free from blemishes, bruises and be a beautiful deep, bright red. A pale or dull tomato is going to lackluster. Avoid discolorations, a little black spot could indicate rot on the inside.
As with many fruits, tomatoes are often picked when they are still green to make it through transportation time. This makes them less tasty. Often they will ripen during transit, yet some tomatoes are spray treated with ethylene gas to speed ripening. This artificial ripening produces bland tomatoes. Thus to get the best tomatoes at the store, look for the term "vine-ripened" or alternatively visit a local farmer's market.
Second, feel the tomato. How is its weight? How much does give when squeezed? A healthy tomato can resist pressure, but is not so hard that it does not react to the touch. Check for soft areas, soft spots mean that there tomato will soon spoil.
Lastly, smell the tomato near the stem. If it's good it should have a sweet, strong earthy odor. The greater the fragrance, the more flavor the tomato will possess. Keep clear of tomatoes with no smell.
Needless to say, organic AND local tomatoes are the healthiest options for a variety of reasons.