To get the list of predefined macros, run the following commands.
touch t.c
gcc -E -dM -c t.c

The compiler will dump the list of predefined macros. Here is a sample:
#define __DBL_MIN_EXP__ (-1021)
#define __FLT_MIN__ 1.17549435e-38F
#define __CHAR_BIT__ 8
#define __WCHAR_MAX__ 2147483647
#define __DBL_DENORM_MIN__ 4.9406564584124654e-324
#define __FLT_EVAL_METHOD__

Note: Only tested with GNU GCC on Linux and OpenIndiana.