Can I find what #defines's the pre-processor makes?

[Dr. David Kirkby]

I know if I use gcc, it will define __GNUC__, and no doubt many other things 
too. Does anyone know how to get a list of them all? Is there a command I can 
execute which will list them?

I'd rather a command, than a pointer to documentation, as I want to do this on a 
gcc-like complier from Sun, which has a gcc-front end, but uses Sun's superior 
back end, to generate faster code. I want to know what that defines, but since 
it acts like gcc, anything that works on gcc should work on that too.

Dave