To attempt to identify the sex of your new kitten, first stand "it" on a table, in a good light and lift up the tail.
In the male the uro-genital opening is a small circular dot below and slightly separated from the anus, rather like a colon (:). it may also be possible to see the slight swelling of the testicles, just below the anus.
In the female the opening (vulva) is in the same situation, below but closer to the anus and is more elongated vertically.
When looking at a male and female kitten together the differences are fairly obvious but considering one on its own can be puzzling, especially when they are young.