libpq -- reading a timestamp with time zone using binary format

how does one determine whether libpq is sending an int64 or a double?

I see all of the #ifdefs in the source:

#ifdef HAVE_INT64_TIMESTAMP
static int64 time2t(const int hour, const int min, const int sec,
const fsec_t fsec);
#else
static double time2t(const int hour, const int min, const int sec,
const fsec_t fsec);
#endif

but it is still unclear (at least to me) how to determine as the
client whether the server has been compiled with the
HAVE_INT64_TIMESTAMP flag.

since both int64 and double use 8 bytes of storage on x86_64, you
can't determine the type using PQfsize.

Any suggestions would be appreciated.

-Whit