The encryption uses LUKS with dm-crypt (not plain dm-crypt).

Quoting http://code.google.com/p/cryptsetup/wiki/DMCrypt:

cryptsetup utility support several modes. Plain mode is just equivalent of direct configuration of dmcrypt target with passphrase hashing but without on-disk metadata.

LUKS (Linux Unified Key Setup) is now the preferred way to set up disk encryption with dm-crypt using the cryptsetup utility

It's more likely that Ubuntu uses the recommended and more secure way of disk encryption. Now, to verify so, read the contents of the disk (assuming that /dev/sda2
contains your encrypted partition):

sudo dd if=/dev/sda2 bs=512 count=1 | xxd

You'll see several options, such as the cipher being used and the ID of the encrypted partition.

Side note: if you only have one partition to encrypt, I suggest you to avoid LVM at all and use LUKS only (which can be done with the manual partitioning method). This avoid the overhead of LVM which you don't need for just a single partition. On the other hand, if you have multiple partitions to encrypt (/, /home, swap), LVM on the top of LUKS is more convenient as you have to enter your passphrase only once. (physical - LUKS - LVM - /home, /, swap, etc)