Tomatoes come in varieties with different grow habits. The fruits can be grown on plants with a determinate, indeterminate and even semi-determinate manner. The manner of growth will determine if and how you will be pruning the plants. Pruning tomatoes is not necessary, but may increase the size of fruit marginally and will make the plants easier to manage. Pruning usually means removal of suckers and occasionally pinching back to enhance compact growth and stem strength. The difference between determinate and indeterminate ultimately boils down to the length of harvest and the time of harvest. Determinates produce fruit first, usually in a two- to three-week period, while indeterminate varieties will produce fruit for a longer period, but near the end of the summer.
Look at the branching formation on your plants. If the branches are long with sparse foliage, they are likely indeterminate. A compact, bushy plant is determinate because it form flowers at the terminal end of the branch, which signals the stem to stop growing. You will need to stake an indeterminate plant or it will fall over when it bears fruit. Determinate plants rarely require staking.
Check for suckers at the base of your tomato plant and at the crotch of a leaved stem. These are small growth that will increase density in the tomato plant's crown, but will not flower and fruit. They need to be pruned out and they only happen on indeterminate plants. If they are left to grow, the plant may have mildew problems due to lack of air circulation, problems with fruit ripening because sunlight is not penetrating, and the suckers draw energy that would be better used on the fruit.
Consider the height of the plant. Determinate varieties are usually only 2 to 3 feet tall. Indeterminate plants can get 6 feet tall and have numerous long branches. The determinate plants are also referred to as compact and usually have smaller fruit than indeterminate varieties. Some of the newer varieties of tomatoes are determinate while the older ones and many heirlooms are indeterminate.
Count how many flowers are in various stages and how many are simply mature and ready to fruit. The determinate plants will flower nearly all at once and set fruit that will be the one crop on the plant and will ripen at nearly the same time. Then the plant yellows and is finished. The indeterminate plant will have buds, flowers and flowers that have been pollinated all at once and the cycle will continue all season long.
Check the plant tag for your variety. Some of the common indeterminate types grown in the home garden are: Brandywine, Early Girl and Mr. Stripey. Some determinates that have proven to do well in the home vegetable garden are: Celebrity, Small Fry and Oregon Spring.