C, 66 48

Original:

int l(long*s){return strlen(s)/((*s&~(-1L<<56))!=0x656c676f6f67);}

Using OSX gcc,
l("duck"); returns 4,
l("google"); causes Floating point exception: 8.

On other platforms, the constants may need to be adjusted for endianness.

Shorter:

less trickyness, same error.

 l(int*s){return strlen(s)/!!strcmp(s,"Google");}

C, 66 48

Original:

int l(long*s){return strlen(s)/((*s&~(-1L<<56))!=0x656c676f6f67);}

Using OSX gcc,
l("duck"); returns 4,
l("google"); causes Floating point exception: 8.

On other platforms, the constants may need to be adjusted for endianness.

Shorter:

less trickyness, same results.

 l(int*s){return strlen(s)/!!strcmp(s,"Google");}

C, 66

int l(long*s){return strlen(s)/((*s&~(-1L<<54))!=0x656c676f6f67);}

Using OSX gcc,
l("duck"); returns 4,
l("google"); causes Floating point exception: 8.

On other platforms, the constants may need to be adjusted for endianness.

C, 66 48

Original:

int l(long*s){return strlen(s)/((*s&~(-1L<<56))!=0x656c676f6f67);}

Using OSX gcc,
l("duck"); returns 4,
l("google"); causes Floating point exception: 8.

On other platforms, the constants may need to be adjusted for endianness.

Shorter:

less trickyness, same error.

 l(int*s){return strlen(s)/!!strcmp(s,"Google");}