What difficulty is this save/film played on?
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So someone sent you a film where he kills all 30 Trow on River of blood. So you want to know what level he played. Well simply put, check out offset hex 75 into the file. This is one of

00=Timid.
01=Easy
02=Normal
03=Heroic
04=Legendary

Too my knowledge, difficulty has no impact at all on multiplayer games.

You can use a hexeditor to check this or use one of these utilities that checks the difficulty - Quick and dirty and Mac only for now.

Download Mythdifficulty (Binhexed 68 K) Drop your saves or films on the program. It will "check" any program, there no validation if its a myth file.

NEW Here is an Applescript that checks the difficulty. It was sent to me by Vodi. I am really impressed by this, I had no idea Applescript could do such things.

Download Vodi's Applescript. (Binhexed 12 K) Drop your saves or films on the program

Another way of getting an idea of the level, for people without hexeditors.
Timid/Easy - Enemy units do less damage.
Heroic/Legendary - More enemies. Sometimes less friendly units. (18 mar Thanks to Poena Dare)

More on difficulty on multiplayer 18 mar:
King Jeff says that units does more damage on higher difficulties. I remember a discussion on alt.games.myth where Dorian Ouer (who is Bungie's representative on that ng) claimed that this was so on the betas, but it isn't so any more.
I have tried running multiplayer films at different difficulty levels, but noticed no change. The investigation continues.