Typically fruit smoothies are good for you. Fruits are good sources of folates, vitamin C, and potassium so fruit smoothies are packed with these beneficial nutrients. You can also get protein and/or calcium from your smoothie if you have included yogurt or milk in it. Basically the real way to know if the smoothies you are making are good for you depends on what the ingredients are.
Smoothies that are compiled mostly of just fruit and a splash of a liquid like milk or even water are better for you than smoothies with added sugar. You must also be conscious of your serving size as well. You must stay within a recommended serving to be getting the real benefits from that you need from your smoothie without getting too much of certain components.