I think that fdisk will respond with 83 for ext2, ext3 and even Reiserfs.
First, if necessary use fdisk -l to check if the partition is a Linux partition. Then use "sudo file -s /dev/<dev>".
jschiwal@hpamd64:~> sudo /sbin/fdisk -l /dev/hda
Disk /dev/hda: 100.0 GB, 100030242816 bytes
255 heads, 63 sectors/track, 12161 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Device Boot Start End Blocks Id System
/dev/hda5 3983 3994 96358+ 83 Linux
jschiwal@hpamd64:~> sudo file -s /dev/hda5
/dev/hda5: Linux rev 1.0 ext3 filesystem data (needs journal recovery)
You could also use the "dumpe2fs" program to print out information on the filesystem. If it indicates a journal and journal size, it is ext3. If you have an ext2 filesystem, you can use tune2fs to add journaling, making it ext3.
Just some trivia. After the ext2 filesystem support was included in the kernel, Linus decided that any new features would be included in a new version instead of having different versions of ext2 to deal with. This made ext2 universally compatible. Now that ext3 support is included in the kernel, any new features will go in ext4 which is under development.
This way, an ext3 filesystem will be able to be mounted in an older kernel even if it was created on a newer one.