/proc/PID/exe is a symlink to a filename, not to a file. So you can't tell just by looking at it whether the executable stored at (eg) /usr/sbin/apache is really the one that is running. Better, I think, to do something like this pseudo-code:

foreach PID
  lsof -p PID | grep FILENAME
    and parse out the inode number
  stat FILENAME
  do the inodes match?
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still not foolproof, of course. It's also far more portable to use utilities like lsof than to grovel around in /proc which doesn't even exist on some systems. But on Linux/x86 (and presumably Linux/others too) all the information you need is in /proc/PID/maps.