Carbs are listed on food packaging. Fiber, which is a carb, can be subtracted from the total since it cannot be digested. The result is call, net carbs.
For fruits and vegetables, I've looked up their net carbs. There isn't that much variation from week to week with the produce we get so it isn't too big of a burden to memorize them. Some veggies, like lettuce, I treat as free-bees.
The challenge is with soups, stews and other foods that are a combination of other foods. With those foods, I just guess.