only one bit out of sixteen true

Howdy, this isn't really a c++ specific question, but people here know their stuff.

I'm trying to make up a speedy way to determine if one and only one bit in an unsigned sixteen bit int is true.

I know I could iterate over it and test it thusly

unsigned short int special_happy_word = something;
int accumulator = 0;
for (int i = 0; i < 16; i++)
{
  if (special_happy_word & (1 << i))
  {
    accumulator++;
  }
}

if (accumulator == 1)
{
  // ok
}

It wouldn't really be slow, but I'll be doing this very often and I'm wondering if anyone knows of a faster way.