I've found this trick on the net sometime in the past..

v = the_byte_value;
v ^= v >> 4;
v &= 0xf;
parity = (0x6996 >> v) & 1;

of course, the 8051 isn't too efficient with shifting or 16-bit values, so the most efficient way is to move the wanted value to accumulator and reading parity from psw..