Well this is going to sound like an annoying answer, built in objects are ones you don't have to instantiate, you access the object directly. Built in objects are made by Flash.
Obviously, built in functions are exactly the same, in that we dont have create them, for example, you know what the
function does - you know that you can always use that, because you know it is a built in function and will always be available.
Its built it - its very easy to tell built in objects apart from user defined ones - cos you have to make the ones that aren't built it, simple !
You could also change your settings in Flash so that whenever you use a built object or function it appears bright pink - then you'd know.