> I try: *p==0 && p[sizeof(double)-1]==0 && p[(sizeof(double)-1)/2]==0;

Sure, that should work.  It would seem cleaner and safer to use copysign, 
though: that way, things will still work when some other byte layout comes 
along, or when some version of Python starts using 128-bit IEEE 754 
doubles instead of 64-bit, or ...

Reopening:  I've been meaning to fix these checks to use copysign for a 
while now, anyway.