But pro-logic (unlike pro-logic II) is not intended to be used on a stereo source that was not surround-encoded. So if you can't rely on the presence or absence of a logo on the packaging, and it's not something like a regular movie that you can use IMDB to tell whether or not it was originally a Dolby stereo mix - how do you know?

Actually, let's use an example: "Walking With Dinosaurs" , the 1999 BBC TV series - the DVD packaging says Dolby Digital 2.0 stereo(despite there being a 5.1 trailer on the disc!), IMDB says stereo, and yes, the audio is in stereo. However if you turn pro-logic decoding on whilst listening to this, it sounds like the audio was deliberately encoded with surround information. And it sounds more to me than just unintentional out-of-phase like you would get when listening to a music CD with pro-logic turned on.

Now, what about the French soundtrack of the Star Wars laserdiscs? Well, according to the LDDb, "the sound of this LD is Stereo and not Dolby Surround because the original 1977 French mix for the movie was only a two-track Stereo mix, due to limitations of French theaters at the time". So if this is correct, and I was to sample and encode the French track for a DVD, it would be wrong to set the surround flag in the AC-3 stream. Is there any way to confirm this?