Because clone Bruces grave was robbed at the start of the event and Barry and Hal were investigating it... theres only so many random skulls that would show up on the same day a grave is robbed and only the skull is taken.

True...Edit: I should have said Bruce's cloned skull which it likely would not have had any of the telltale scars from when his skull was damaged by Hush as it was a clone...likely would not have had any of the bats sca...rs...wait that could have been a tip-off to red robin that it was not Bruce Wayne who was buried in that grave. bones tend to heal thicker when broken and if Bruce's vertebrae and skull did not have the calcification scars on them, that would have tipped off someone who was paying attention.

Have yet to read crises that leads to the apparent 'death' of batman yet...mainly because of the fact that it has the apparent death of batman, and batman was a big part of my growing up and I do not like the idea of him being killed off...yet love to see superman killed off or at least get butt handed to him, as I dislike him and tolerate him as he is are deterrent to guys like Mongo and Darkseid.

Thats actually a good detail. I suppose nobody ever inspected his body that detail then.

To be fair, Final Crisis had its fair share of issues other than killing Batman. Only the tie in stuff was really interesting.