For us, those affected by diabetes, it's all about the carbs or carbohydrates. We don't care about the calories nor the fat content of the food (well some of us do but not for the insulin purposes). In order to calculate how much insulin is needed to cover for the food, and ultimately keep a balanced blood sugar, we mainly need to know how much carbs it has per what is eaten. That can be pretty complicated but with simple tools and some experience it can become almost automatic. Some of the suggestions can also be used for those counting calories as well, but in many cases the calorie content doesn't give any insights about the carb content and vice versa; a certain food can have a very high fat content and so very high calorie index and yet might have no carbs. Other foods might have a low fat index but still be very reach in carbs. In general there are several approaches for carb counting. Some prefer educated guess work based on portion sizes and food groups, that can make life easier for some but I find it quite confusing, others prefer memorizing carb content or eat fixed dishes at fixed content (doesn't work so well when it comes to kids or if you are a spontaneous type). And some prefer a more scientific approach, I find myself within this group most of the time, maybe it's the scientist in me that takes over. It requires some preliminary work but it really helps you get a very accurate carb count for the food.
A great tool that can be used for eating outside and can be great for all of the schools above (and also for those counting calories) is the app 'Nutrition Menu' by Shroomies, I have it on my Iphone and one can find most of the known restaurants on their list and check out their menu and the nutritional value including carb content for most food items, very user friendly and certainly a well spent $0.99.
Another nice app that I often use, that was actually recommended to me by my dad, is 'My Fitness Pal', it has a great list of many store bought food items from many different brands with their full nutritional value. This app also helps you keep track of your activity levels, food consumption and content and even weight and exercise plan and management for those interested in that feature.
Now the other thing I have found to be very useful is the carb factor. Every food has its own carb factor that can be calculated by dividing the carb content of a portion by it's weight. For example: on a box of cereal you can see that 28gr of these cereals equals 24gr of carbs. Now 24 divided by 28 equals 0.86, and let say you eat 50gr of that cereal brand then 50 multiply by 0.86 equals 43 gram of carbs. By the same way I calculated the carb factor for cooked pasta 0.279 or for most bread like baked goods 0.5, or for any other food my daughter likes. This way I can weigh the portion she eats and calculate exactly the amount of carbs it has. I used to also carry around a small scale in my bag, but I decide that sometimes one needs to let go, so for cases we are not at home I use one of the previously mentioned apps and that usually works pretty well. Now, interesting enough, my husband actually doesn't use any of these fancy techniques, he prefers the intuition way, or the hunch. He looks at the food and gives an estimated number, and when compared to the other ways, his guess is usually more accurate than mine, but I guess it requires special skills that I am yet to acquire.