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

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

#define CAT(a,b) a ## b

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

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

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