这是行业标准:

int is_negative(float num)
{
  char *p = (char*) malloc(20);
  sprintf(p, "%f", num);
  return p[0] == '-';
}

对于32位有符号整数，例如System。Int32，也就是c#中的int:

bool isNegative = (num & (1 << 31)) != 0;

bool isNegative(int n) {
  int i;
  for (i = 0; i <= Int32.MaxValue; i++) {
    if (n == i) 
      return false;
  }
  return true;
}

if (num < 0) {
  //negative
}
if (num > 0) {
  //positive
}
if (num == 0) {
  //neither positive or negative,
}

或者使用else if

num < 0 // number is negative

int j = num * -1;

if (j - num  == j)
{
     // Num is equal to zero
}
else if (j < i)
{
      // Num is positive
}
else if (j > i) 
{
     // Num is negative
}