Is this the pic you were looking for?
The easiest way is to look under the tail of each kitten and hold two side by side until you have two that look different. A female will have an upside-down exclamation mark and a male will look more like a colon : Also, the male's urinary opening is lower than the females and he will also have a little lump between his anus and this opening. This will later become his testicles.
Hope this helps.