Originally Posted by Kram Sacul
How do you figure 720p?
I did the usual "Downscaling-Upscaling" Test
. DU Test for short.
If downscaling the original 1080p image to an X resolution and upscaling it back to 1080p it doesn't loose any detail, then the X resolution is likely to be near the original. You can do it yourself with Photoshop using the screenshots posted by Xylon. Just remember to use the best upscaling/downscaling algorithm available in Photoshop.
Here's the DU Test for King Kong HD DVD, a reference movie known to be from a true 1080p master:
You can clearly see the loss of details in the DU process.
And there is the DU Test for Fifth Element Remastered (AVC):
As you can see, there is no loss of detail in the DU process, so the master must be around the 720p resolution (+-100).