Just look at the light switch in the door jam. Round or oval 3 hole
Round push-in light-switches confirm it's a '59, but they were introduced very late - September or thereabouts. Having the diamond shaped ones is by no means confirmation it's definitely a '58.
The most accurate guide that no-one seems to have mentioned thus far is the chassis number - assuming the subframe hasn't been replaced. If it starts with an 8, it's May-November '58. If it starts with FC, it's December 58 onwards. Obviously if it starts with a 6 or 7, it's not original. Otherwise, someone like Ken can usually assert with reasonable confidence if the chassis number looks like it belongs with the body plate ID number.
Why did they change the chassis numbering scheme at that point, I hear you ask? Well, someone mentioned the phrase "She's a Beauty" earlier. There's a clue...
cheers
RET