First please do NOT believe what it says on the front of the box! So many of the boxes that scream "healthy" are not even close to a healthy choice. The food industry is allowed to print all kinds of untrue statements and they lie all the time. It is imperative that you take the time to read labels!
Start with the ingredients. Choose cereals with the fewest ingredients possible and aim for no more than 8 ingredients. All those added vitamins and minerals wouldn't be necessary if we didn't over-process the ingredients and strip them away in the first place.
Next check out the "Nutrition Facts" and pay attention to serving size which is probably a lot smaller than what you actually eat. For fun try pouring a bowl of your usual cereal, then use a measuring cup to compare your actual serving size with what is listed on the box. Most of us eat 2-4 servings at a time. Do the math and see how the calories, sugar, fat and sodium stack up with your real life serving. Even cereal with seemingly little sugar gets pretty unhealthy when you eat several servings at a time. For information about recommended sugar per day see attached chart. Note that the amounts listed are for added sugar and do not include such things as fruit and milk which contain naturally occurring sugars that do not wreck havoc on the body the way added sugars do.
Next check out the sugar. The healthiest choices have 5 or less grams of sugar per serving but no more than 8 grams per serving. Avoid cereals that list sugar at or near the top of the ingredient list .Also be careful that sugar is not hidden under another name and avoid any cereal with high fructose corn syrup as it is a sure sign that it is highly processed.
Aim for cereals that have whole grains as the first ingredient. Don’t be fooled by terms such as “enriched wheat”.
FIber is important, its presence is a good indication that the ingredients are not over-processed. Aim for a cereal with at least 3 grams (g) of fiber per serving, but if possible, aim for 5 grams or more per serving. Avoid cereals with added fiber from ingredients such as inulin which can cause stomach upset.
Most cereals don't offer much in the way of protein but you will get a good serving of protein from cow or soy milk (both are complete proteins). Other options such as rice, almond and coconut milk don't offer much if any protein, so consider adding some nuts or chia seeds to the bowl.
Best advice is to really read the labels and periodically recheck your favorites because companies are continually changing ingredients.
Happy breakfast (or whenever you prefer your cereal)!