I often do not order from the main entree section as I do not eat steaks and seafood but if they have a (whole) pizza or pasta section or sometimes a burger section where you get a full plate of food often accompanied by a choice of sides but not always it is generally an entree. I would not go by the price as I have seen appetizers for similar prices to entrees. They are usually listed as appetizers or starters or sides and unless it is the combo including all the appetizers on one plate it is generally a much smaller portion of food. I have asked the server which areas of the menus are considered entrees. They are usually spot on.
But yes, if your plan is to not order from the official entree list it is best to check first as you are putting out a lot of $ on a fine dining shop. I also generally ask if I can get a cup of soup as my appetizer as it is much less expensive but also not on the official appetizer list. One of these is also usually required.
For dessert, some restaurants will allow you to order a scoop of ice cream which is often not listed on the menu, definitely is dessert and without the brownie or whatever under it leaves you more money for the entree.