The most sure way is by behavior...if you have a male bird you know is male throw her in and see how they act. If it's male they will fight...female she may acted interested in his display... You can also feel the vent area and if the bone is spread wide enough to pass an egg it most likely is female...With pigeons you really can't tell by looks alone
The best way to determine the sex of a pigeon is to ask another pigeon. In a perfect world ♂ are usually more iridescent and larger than ♀ the bird on the right is a ♂ pigeon.
That being said no matter which method you choose to sex a pigeon mistakes are still made. If you place your pigeon in the company of a know gender of a pigeon one can usually tell the gender of the pigeon in question by the reaction taken by the know gender. Check out the link the pigeon bowing and circling is ♂ the bird walking is ♀: