Very carefully and it takes a lot of practice. It's the way the organ looks that the vent sexers look at. Done wrong, it can injure the bird. Best to have an experienced sexer show you in person.
If you want written info for the theory you could try "Sexing all fowl" - but inexperienced vent sexing can damage the chicks. Also there are not 2 distinctive looks, 1 boy 1 girl but rather a variety of different looking "bumps" and it takes time and practice to recognise them. In some countries there are courses offered in the technique, but it is generally not cost effective for other than hatcheries.