How early can we know the sex of a human being now?
Before the zygote, there is no individual for which sex can be defined! However, it is possible to know the sex of the future zygote if you know the gametes that are going to fuse.

In humans, just like in all placental mammals (which excl. kangaroos and platypus), sex is defined by sexual chromosomes, where males are XY while females are XX. A mother can therefore only pass on an X (and have no influence on the sex of the offspring) while a father can pass on either an X or a Y which will determine the sex of the offspring. Knowing the paternal gamete (the spermatozoid), one can infer what the sex of the future offspring will be. Of course, in one ejaculate, there are about as many X-bearing gametes than Y-bearing gametes.

Technically speaking, could typically make a karyotype to infer whether chromosome X or Y is being transmitted.