Here's the important bit from Phil's post:
http://en.wikipedia.o...t_wall_infarction.jpg
It's the presence of the damaged tissue, at the top of the picture, which reveals that the person suffered a heart attack.
Also (although I don't understand why or how) there are rapid changes to some hormone levels in the body when a heart attack takes place. I only know that because a colleague of mine suffered a heart attack which wasn't diagnosed by a first-aider (me!). She just appeared to have fainted and then, after a few minutes, she came round again. The following morning she suffered another heart attack which, again wasn't spotted by a first aider (who, this time, was also a qualified nurse). Again, she just seemed to have fainted but she was taken to hospital as a precautionary measure. The hospital staff also failed to realise that she'd had a heart attack but took routine blood samples anyway. It was only when they saw her hormone levels that she was suddenly whisked into the intensive care unit!
Chris