Shop around. To find a sofa with the right depth, you may need to check several furniture stores in your area. Keep track of the options you find and the prices.
Bring the family along. Once you narrow down your choices, get input from your family members. Have each one sit on the sofas you're choosing from and let you know how they feel about the depth.
Factor in growth. Remember that small children will likely not fit well on a sofa with standard depth. However, they will eventually grow into it. The adults in your household should make the final decision.
Pay attention to the height of the people who will be using the couch most often. People who are shorter will require less depth than those with longer legs. However, the best way to tell is to have them actually sit on the sofa. Generally, a person's feet should be able to be flat on the floor while his back is straight against the back of the sofa.