How can you differentiate healthy food from unhealthy food?

Healthy typically means fresh, unprocessed or minimally processed foods. Fruits, vegetables, unprocessed meats.

.

Unhealthy (nor simply foods with little to no health benefits) are typically highly processed, full of added and artificial ingredients. High in unhealthy fats and/or sugars. Foods whose ingredient list contains a number of ingredients the consumer isn't' familiar with. 

.

Not all fast foods are unhealthy. However, many are high in fat/calories and have little no no nutritional value. Hamburgers (high fat meat on a white bread bun), friend chicken (breaded with processed flour and fried in oil), pizza (white flour crust and fatty meat toppings, sauce high in sugar), french fries (white potatoes fried in oil and then drenched in salt), and the list could go on. 

Healthy foods are more "natural" - fresh veggies, fruits, meats, and whole grains.  (and you don't have to eat the veggies raw to get healthy benefits.  Cooking fresh veggies is still "healthy".)

The more "processing" that happens to something, the less healthy it becomes.  

Fast foods are unhealthy because they are usually very high in the wrong kinds of fats and usually have too much added salt.  They are also very high in calories and low in ACTUAL nutrition.  

Unhealthy foods are typically high in salt, fats, carbs, sugar, and unhealthy cholesterol.  Those elements are abundant among fast foods.

In their natural state (fruit, vegetables, deli meat) vs processed food. Obviously not EVERY processed food is less healthy, but the vast majority are.