One test I like to use is the begining titles of Lethal Weapon 3. The Eric Clapton song sounds far superior in DTS. It sounds like flat stereo in DD while it fills the room with excellent imaging in the DTS version. This is where DTS often shines. The imaging is superior and the bass is often tighter.

However, a well mixed DD track can still sound amazing. Look at Terminator 3 or The Lion King. That is the beauty of remixing for HT.