Is there a recommended way to determine what platform a configuration is being generated for?

I'm kind of hacking around it at the moment but when building natively on windows, I would like to set defines for x32/x64 except if I say:

configuration "x32"
defines( { "somedefine" } )

it doesn't seem to show up, it does if I say configuration "windows" instead but that confuses things if I'm cross compiling as well.