It's not impossible to tell. Diabetics and others with dietary restrictions, have to do that sort of thing every day to decide what they can and can't eat. If you pick up a loaf of bread at the baker's, the baker usually has no idea the amount of carbs, or salt, or sugar is in that loaf of bread. Try it. Ask the baker: "how much carbs in this loaf of bread?". They'll just stare at you. They can tell you whether the loaf has salt or sugar in it but can't tell you how much because they don't calculate the amount by loaf. So if you have dietary restrictions you have to know what you can and can't eat and that requires the ability to judge the contents of a piece of food from the way it looks.

Photons don't carry that information? Sure. But you don't just have photons to go by. You can rely on a large database of prior knowledge about how food is usually made and with what ingredients.

Other people who have to rely on their imperfect human senses to decide what they can and can't eat: people with allergies, people with heart problems, hypertensives, kidney patients, etc. etc.