There are casting numbers on our blocks, but the 5.0 OEM block hasn't changed for a couple of decades, I believe. The OEM GT cylinder heads were also used on the earlier models as well, if I remember correctly.
If you're trying to decide if a car has the engine changes that a seller says it has, that can be kinda rough to tell. Many sellers will lie through their teeth to sell a car, sadly. While you can tell if some parts have changed, you can't always tell everything. It is difficult to describe what to look for; can you give us more info on what you're looking for? Like, are you looking to buy a Mustang and the seller says it has XXXX parts on it? Or do you already own the car, but you think it has an aftermarket cam?