For a 2.39:1 scope movie, that means the total width of the screen covers just a hair over 60 degress of my vision. Going by the 1 arcminute figure, that means a digital image needs 3600 pixels across its width to appear smooth. Anything less than that will exhibit jaggies. This is confirmed by the fact that I can often see stair-stepping in 2K resolution f/x shots and 1280x1024 DLP presentations.

As John P. noted, it can be easier to spot these artifacts in bright and high-contrast scenes. So if you're still having trouble seeing the pixels, look closely at those parts of the picture. And if you still can't see them, even at 2 screen heights' distance, either the projector is out of focus, or your vision may need correction.

Note that a scope image projected at current DLP resolution will still have visible pixels for viewing angles as small as 22 degrees, so you should be able to see the jaggies, at least in the horizontal direction, even near the back of most auditoriums.