I want to start off by saying that i do not own pigeons and i am gathering this information from this site and others!
Determining the sex of a pigeon can be very very difficult. Even after all this information you still may get the sex completely wrong. Their looks don't often give away their gender. In some cases though, pigeons looks can be the give away. In the next photo the pigeon on the right is the male. He is more impressive looking than the female and is slightly larger.
Another way that people sex their pigeons is by waiting until their pigeons lays or not. This can be a long process though as pigeons can begin to lay anywhere between 5 months and 18 months. Many people though don't like to wait that long.
The third option is to put the pigeon in with a definite male pigeon. Usually if the pigeon is a male then they will usually fight but if it is a female, then she should act interested in his display. This can be risky because if it is a male then they could end up hurting each other.
The last option is to feel your way down the pelvic bone to the vent area. The more closer and tighter it is together the more likely it is to be a male. The more space there is between the bones, the more likely it is to be a female. Even if it is a female that isn't laying there should be some give when you apply gentle pressure to the bones!
Again even using all these methods, it is almost never 100% accurate unless they lay eggs!