I read somewhere that after the two Na'vi mate there is a bond between them that can actually be seen sejak other Na'vi. This is how he knew they mated. I cant remember where I read this unfortunately but i will look to get anda a link.
The male and female Na'vi will connect Tsahaylu to create an emotional bond that lasts a lifetime. Once Tsahaylu is made between the couple, the ultimate in intimacy,this causes the somewhat unwillful sharing of the couple's good memories, and is a sign of Eywa's acceptance.