It's fairly easy to tell if a meal is bad for you using basic nutritional knowledge. The idea I think is best explained through a reference to a Simpsons episode wherein Homer goes on the "clear" diet, eating only (greasy) foods that turn things like napkins--and, incidentally, entire walls--translucent. In the real world we have nutritionalists, and an interesting Web service called MyFoodPhone that is best explained as a weight-loss and nutritional-education program slash social network.
The premise is simple: Just take photos to log each meal you've eaten and send them to the service via MMS. These entries can be added using your phone, or by logging them on the Web site. You can then monitor your progress on a chart, which can also contain biometric data if you put in the time to enter it on a daily basis.
From there the service branches off in two directions: community, and a paid expert service. With the community section you get your own team. It's a small group composed of other users who are tracking their meals and goals. It's essentially an online support group designed to make you feel responsible for your eating habits.
The other end, and the far more interesting one, is the paid service. Ten bucks a month gets you your own nutritionalist who will go over your habits and give you feedback, including recommendations on what you could and should be doing better. The paid service also gives you access to nutritional videos, which have been hand-picked by your adviser for you.
What's fascinating is the expansion beyond this service into the medical field. Parent company Myca is working on two other services (DoctorPhone and BabyPhone) aimed at helping people get medical advice using their camera phones. In theory, you'd be able to call and ask a question, or take a picture of something you're concerned about (use your imagination on that one), and get hooked up to a nurse or doctor to talk you through it. According to an article by Business 2.0 Magazine, Myca is working with the insurance industry to get these services covered by people's pre-existing health plans, which is fairly attractive assuming your current health-care provider doesn't have good phone-nursing services.