Pay attention to what other people buy
Caution is required for this one, or you risk being labelled as a stalker or being thrown out of the supermarket by security. This kind of thing can get you a reputation. Start watching what other people buy, and how they behave in the supermarket.
Most of us assume that everyone shops the same way, but they don't. Under the guise of "minding our own business" we go into the supermarket and do our thing and leave. But if you start looking at what other people do, you will see some interesting behaviours and choices emerge.
For example, in the really large supermarkets watch the cereal aisle. Overwhelmed by choice, people tend to locate their usual brand as quickly as possible and leave the aisle. Few people take time to see what other cereals are on offer, what their ingredients are, or what cereal alternatives there are.
You might see some people load up their trolleys/carts with dieting or slimming foods, and then put in ten 2 litre bottles of fizzy sugary drinks. Worried about their blood pressure some people will buy low sodium salt... and then go and buy their cartoon of cigarettes.
When watching other people, these incongruencies are easy to see, but when it comes to our own choices; they often come out of our own blind-spots.
Another common behaviour is the choice between tinned and fresh fruit and vegetables. The tinned version are far from fresh, often come laden with added sugar and salt and are more expensive. But in the name of convenience, these may be bought in volumes. "Eat more vegetables" often gets interpreted as, "eat more tinned vegetables." This is not a good choice.
Convenience ready meals / TV dinners are popular choices too. With questionable nutritional value, these are marketed to the overweight with the offerings of "low fat" or reduced calories. Some people will try to off-set this by adding vitamin supplements to their trolley/cart.
Supermarkets overwhelm us with choice, and in a way are able to direct and control our choices to what is profitable for them. Remember, supermarkets exist to make money and profit. There is nothing wrong with this and they provide a good service, but at the end of the day, the reason they are there is to make money.
So, slow down in the supermarket, look at what else is offered, watch how other people make choices to what they buy and thus subsequently put into their bodies.