I can't. Especially kittens. We have a a male whose original name was Emma and we changed it to Ricky. Then a male named Popsicle that we thought was a girl, left him Popsicle since it's rather neutral gender-wise. And his sister Nellie, who turned out to be brother. He's still Nellie.
No. I have a spayed female who's head is more the size of a male, and a neutered male who has a smaller head like a female. I've been fooled a number of times and so have some of my breeder friends. Generally speaking females are smaller all around, but not always.
Unless you check to see if a cat has the family jewels or not is the only way to tell whether a cat is male or female has been my experience after TNRing over 800 cats in our TNR group. large heads or size are not a true indicator of the sex of the cat.
Type of cat may also help you determine if the cat is male/female: according to a person whose full time job is taking care of feral cats and cats with special needs, orange tabby cats are 80 to 90% male; calico cats, if male, are usually sterile (though I believe most calico cats are female).
The orange tabby we TNR was male.