If the method is simply returning an attribute of the object, I would call it an accessor, since it is simply used to access a private member variable of the object.
If the method performs some kind of computation or contains logic (beyond the possibility of logging or validation), it's simply a method. It's no different than a method that performs any other operations that use the state of the object and return a value that's not a boolean.
If the method preforms some special task, such as checking equality or validity, I might want to refer to it by what it does, such as calling it a "validator method" or "equality method". Those names is a bit shorter than saying "a method that checks the validity of the object" or "a method that checks the equality of this object against another".
Not everything has a special name, though. Call it by what it is.