In the US, the backscatter x-ray machines are two giant ugly blue boxes that you stand between. The mm wave machines are more futuristic-looking, white cylinders that you stand inside of. (photos here http://www.flyertalk.com/forum/practical-travel-safety-issue...)

The technology deployment is airport-specific, so you will only find one or the other at any given airport, e.g. O'Hare uses Backscatter, Midway uses mm wave.

La Guardia is the one airport serving a major US city that does not have any of the new scanners, which can be nice if you typically opt-out.

Even if there is, I'm guessing it would be hard to tell at a glance whether the machine was calibrated by someone with a physics degree or someone who'd be delivering pizzas if it weren't for the growth of the security industry.

I can in no way substantiate this, but the xray ones are usually two large (maybe always blue?) mostly rectangular blocks that you stand in between. The millimeter machines are usually rounder and have clear bits. The illustrations here might help: http://www.jaunted.com/story/2010/1/5/163631/3181/travel/Ful...

edit: never mind, the flyer talk pics are much better.