Even when restaurants provide nutrition guides they are often times laughably inaccurate for a multitude of reasons.
Sometimes it is down to serving size. For example all chicken breasts aren't the same and you don't always get the same size scoop of pasta or the same amount of sauce. Along those same lines many times the nutritional information on that plate of pasta might be for a one cup serving even though you are getting 3 cups of pasta on your plate.
Chefs also experiment. The Nutritional information might be for preparing a dish with olive oil but the chef finds the dish is liked better if it is made with butter. Chefs like to play with recipes and the same dish from one day might not have the exact same ingredients the next.
I think it is much better instead to know what you are eating and use known values to roughly calculate the nutritional information. With the abundance of smartphones it isn't all that hard. If you get a chicken dinner that is a grilled chicken breast, a side of broccoli, and a salad you can make an estimate on the calories if you can estimate the size of the serving and look up the nutritional information. A cup of broccoli will have the nutritional information of a cup of broccoli. A 12 oz chicken breast will have the nutritional information of a 12 oz chicken breast. It is important to know if that chicken breast is covered in a creamy sauce of if the broccoli is cooked in butter but that is easy, ask.
People look at the nutritional information at a restaurant and assume it is gospel, it very much is not. There are too many variables in serving size and preparation for it to be.