Even better, each ext2fs partition has a superblock with a magic number in it.
Some time ago, I wrote a program called findsuper that looks for sectors
containing this magic number in the right place. Last I checked,
findsuper was part of the ext2fs distribution. (I can e-mail it out if
you can't find it.) Findsuper can find your ext2fs partitions, even if
you have no clue what your partition table should look like.
Steve