As young birds it can be tuff, unless you obviously have a big bold cock bird, generally the cocks are bigger than the hen, the cocks have a flatter head, hens more rounded, As they get older 99% of pigeons you will be able to tell the sex of by there behaviour, But theres always the exection, either small cock you thinks a hen or big hen, you thinks a cock, I had them 4 years and still get it wrong sometimes. (not often though ) :-)