From trying them today -
EDIT - I haven't seen this mentioned and it'd be really useful for a lot of folks - If you have one, put a fast glass like 85mm 1.2 or a 50mm 1.2 and see what the focus is like.
There's a thread about focusing screens - excluding the manual focus folk, it'd be useful to see how the 5D responds to fast glass - can it consistently shoot? They should let you change lens, choose the right/quieter queue camera or sound like you're interesting in buying and they'll push a sale and maybe let you try it. Should know in 3-4 weeks, but this is a great opportunity!
The 1DX and 5D MkIII are on different stands, on metal security cables.
You're not going to get a card sneaked into the 5D i'd imagine seeing as there will be a few Canon employees a foot or so away from you. Not sure about testing the weather sealing either!
After hearing all the complaints about price - yes, it's high, but then i've heard similar at a Jessops about the 70-200 f/2.8 - and photographers still buy it.
There was a lot of folks eyeing up buying it too - the dealers are making brisk sales of the model it seems with preorders.
Fun thing definitely worth trying to see some of the Canon talks if anything appeals - http://www.focus-on-imaging.net/news/?cat=5
(Canon talks halfway down).
Histogram - is it outlined?
I'd take Neuro's advice - take a high ISO photo and look at it zoomed in.
Maybe take shots of the lights and do the same and see if you get black halos?
Q about if movie crop feature is available.
Q about f8 focusing (it's unlikely they'll comment on it i'd imagine though)
(it's pretty evenly lit and fairly well lit in the NEC - I didn't take a meter reading, but it wasn't dark feeling).
You might get away with a HDMI cable (to see if you plug it in during live view if the LCD goes out or stays on - presumably that's not needing to attach anything to the other end jlev23?) - as it's much more unexpected than trying to sneak or ask to put a memory card in. You could either bring it up suddenly or maybe talk about video a bit first.
High ISO video might be doable.
best bet about making a dark scene - you're going to need to create your own.
For anyone going - maybe the trick is to sneak a ND filter on then test the high ISO shots and high ISO video!
PS - Have a play with the silent mode. If you're used to sound, and the sound of the 5DM2 its fun to (not) hear!
It's an upgraded 5D. Pricey, but think of that as a 1 time cost that you get benefits from every time you use it it seems