Cats......

The last poster was incorrect. Not all orange tabbys are males, but most of them are.
Determining the gender of a young kitten can be much more difficult than that of an adult cat. Male kittens can take quite some time (up to 10 wks of age) before their testicles bcome undecended, so their ''rear ends'' will look a lot like females too.
This is a good reference: http://thepetcenter.com/exa/kitx.html

Well, of course, you can tell by the one special body part that only boys have. Also if you have an orange tabby it means that it is a boy because all orange tabby cats are boys. I hope this helps your problem.

the male cats have bigger genitals

Ummm, to try to put it delicately, look at their behinds. I am not trying to be a smart aleck. The one with testicles is male, the one with two openings is female. You may have to look very closely. They do not generally like this, so be prepared for a fight.

Determining the sex of your kitten can be hard because the genitals looks similar and both male and female have nipples. I put the link that explains it below.

If a boy cat is not neutered, it is very easy to tell, otherwise it is pretty hard, I would ask a vet.