You can easily check if TRIM works for a "normal" ext4 partition: https://askubuntu.com/a/19480/5920.
How to do that for a LUKS-encrypted one? Let's assume the default LUKS setup made by 12.04 Alternate installer (i.e. the one with LVM involved).
Update
What I'm asking here is how I can check that the block on the disk is actually filled with zeros after removing the file, if the file is stored in an encrypted volume.