Conversely, another complicating factor is that not everyone needs the same things. There are diets high and low in fat, carbohydrates, protein, calories; diets without meat, animal products, or whole families of plants; and so on. One standard cannot serve everyone.
There are some basic concepts, though...
* Unprocessed foods are healthier than most minimally processed foods are healthier than ultraprocessed foods. By that rule alone, the less processed a restaurant's menu, the healthier it probably is. Just be aware of the small number of cases where minimally processed foods are better than the unprocessed version, like fermented foods and nixtamalized corn.
* Single, whole foods tend to be healthy. Things made with a lot of ingredients tend to be less healthy. A restaurant with many single-food offerings (grilled trout, steamed snow peas) tends to be healthier than one whose foods have lots of complicated stuff in them, especially manufactured stuff. If you want to dress them up, many of these can be changed from one ingredient to two or three at the table -- which is why fish is often served with butter or lemon, salt and pepper are on the table, etc. It also counts if you combine several whole foods (e.g. in a salad) without adding processed ingredients (such as commercial salad dressing).
* Closely related to the above, the more flexible a menu is, the more people it can serve a healthy meal. Restaurants that rely heavily on premade products have much less ability to customize for special needs compared to restaurants that make everything from scratch. If you're making a salad dressing, it's simple to leave out the pepper; if it comes from a bottle, you can't. Think of how a nice Italian restaurant puts olive oil, salt, pepper, and cheese on the table so you can mix your own bread dip or salad dressing.
* Subtle but powerful: the more seasonal and local a food, the healthier it's likely to be. This is because other things have to be transported and/or preserved, which tends to reduce both their flavor and their nutrients, sometimes dramatically. Restaurants that serve many seasonal, local foods tend to be healthier than those reliant primarily on preserved, long-distance foods. Even within a single category, compare a frozen burger with fillers and preservatives to a wad of fresh-ground cow.
* Lots of things are beneficial in proper proportion but problematic in overload -- fat, carbohydrates, protein, fiber, salt, calories, etc. Too much or too little is bad. But people don't all want or need the same amount. So rather than try to limit all foods, instead offer a range of each. High, moderate, and low will let diners choose what they want. If everything is low-fat, anyone on a high-fat diet or burning 3,000 calories a day is screwed; if everything is high-fat, everyone on a low-fat diet or burning few calories a day is screwed. A restaurant with a diverse range of food composition will make it possible for more people to choose a "healthy" meal based on their needs, compared to a restaurant with a narrow range.
* If you're worried about how healthy a restaurant's menu is, test it by attempting to assemble meals for various diets. How many vegetarian or vegan meals can you make? How many low-calorie vs. high-calorie ones? What if someone needs to avoid salt, dairy, or other ingredients? Can you make a My Plate or LPI? Do they mark the menu so you can tell what traits a given item has? The better a menu can mix-and-match to suit different goals, the more people can enjoy a "healthy" meal at that restaurant.