i want to see if these \"build a . . .\" flash sites (to support a user customized ordering process) are done using regular flash or Flex?
Flash is quite old stuff for building (simple) animations, think of it like a way to create graphic elements and animations.
Flex is a framework for the developers. You can develop applications with it.
A software developer should definitely go for Flex, on the other hand there might be some clicking tools for building Flash animations for people who do not wish to program.
You can hardly distinguish how the sites were built, because the result gets compiled into binary file (you might do a guess based on the file size in some cases)