How can you differentiate healthy food from unhealthy food?

You mean for humans? "Healthy food" is usually in it's natural form, unprocessed and low sugar. It's mostly things that our bodies recognise as food (meat, eggs, fish, vegetables, fruit, animal fats), it contains high amounts of minerals and vitamins. "Unhealthy food" is processed, contains lots of sugars, vegetable/seed oils, chemicals, colours, fillers and thickeners that our bodies can't deal with well, they cause inflammation and don't contain any significant amounts of minerals and vitamins.