#define SECOND(a, b, ...) b

#define IS_PROBE(...) SECOND(__VA_ARGS__, 0)
#define PROBE() ~, 1

#define NOT(x) IS_PROBE(_NOT_ ## x)
#define _NOT_0 PROBE()

#define BOOL(x) NOT(NOT(x))

Should be 0: BOOL(0)
Should be 1: BOOL(1) (oops!)
Should be 1: BOOL(123) (oops!)
Should be 1: BOOL(not zero) (oops!)
Should be 1: BOOL() (oops!)