What's the fastest way to compare sign on a double?

I know that a double has a "sign bit" but I'm not sure if the way I'm "looking for it" in its binary rep is a good idea or not.

Barring "portability" issues, can someone tell me what's going on with this code in MSVC++?

#include <stdio.h>

int main()
{
  double z = 5.0 ;

  __int64 bitSign ;

  __int64 *ptr ;

  ptr = (__int64*)&z ;

  for( __int64 sh = 0 ; sh < 65 ; sh++ )
  {
    bitSign = 1L << sh ; // Weird.  it doesn't do 1.
    printf( "Bit# %d (%llx):  %lld\n",
      sh, bitSign, ( (*ptr) & bitSign) ) ;
  }

}

First, why is starting at bit 32, even though I only shifted by one bit?

Second, is it ok for me to check the 64th bit of a double to check its sign on MSVC++? Or is there a more preferred way?