Some say there is no way that the cub in the end of The Lion King is a female. I must disgree.
1.) The black marks behind the ears have nothing to do with a gender;
Baby Simba - a boy - didn't have them but they appeared later.
Baby Kiara - a girl - didn't have them and they didn't appear later.
Baby Fluffy had them but its gender hasn't been verified by its creators. But judging by Simba, Kiara, Sarabi, Mufasa and Scar, it could be either.
Adult Nala doesn't have them.
But adult Sarabi has them.
Adult Mufasa has them.
But adult Scar doesn't have them. And Mufasa and Scar are even brothers.
SUMMARY; It's not about gender, it's about if the cub happens to inherit the marks or not and if it does, they may or may not have them as a baby. Hence, Simba eventually had them because both of his parents had them. It's not about gender.
2.) The whiskars are about gender, it is even said on the commentary track. But it is also implied that it's so only because they had to draw each whiskar on a seperate blank paper, which must have been ridiculously frustrating. But notice how none of the babies we see has any whiskars, thus we can not tell which gender they are.
3.) The fur colour is usually golden brown or dark brown with males while females are some shade of sandy brown. But since there are various fur colours going around, the colour is not a 100% certain identification way.
So there is no way to identify for sure, the gender of a lion baby in the films. Is there?
The only way to identify the baby's gender would be the genitals which of course has been ditched from the animation views but I'm sure the characters still see them, which is hhow they're the only ones who can know which gender a baby is. Oh, and to them there must also be a certain kind of scent, I'd suppose.
I also don't think Fluffy (the cun in the end) ever had a gender in its creator's view. There was likely no sequel in plans until the popularity of the film, so the cub was there simply to show us that the circle of life starts anew with a new generation. This seems to be the case even in 2003 as they still called it just Fluffy and didn't verify its gender. Leaving Kiara, the daughter from the sequel movie to be its final identity as it's all in the movie universe.