Is the a value I can set a double to that I can test efficiently i.e. not usual double comaprison?

double x= INIT_VALUE;

while (huge loop)
{
if (testifINIT_VALUE(x))
Do this;
else
Do that;
}

For example, perhaps INIT_VALUE could be a negative number - then maybe there is way to detemine whether x is negative by looking at a single byte somewhere?