As crazy as this may seem, this is a rather complicated question. First, what species of swan do you see? If they are Mute Swans, this may be a little easier, but not 100%
reliable. If they are Mute Swans (reddish orange bill with a black knob on top), then males usually have a larger knob and weigh more (i.e., larger in body size) than females.

Now, even if by observing, this may or may not be correct. The most reliable method of sexing is DNA Sequencing through feathers or blood submitted to a licensed veterinary medical laboratory.

Many swan keepers and others may use what is called probing. But, this is the least reliable means if the swans are young birds and the genitalia is not developed. Probing involves the manipulation of the vent to physically sense the presence of a penis or penile node. Again, the younger the bird, this organ may not be visible or felt. If this is the case, then a male bird could be erroneously misidentified as a female and a year later, a new owner is not happy because what the owner bought is not what he or she recieved. So, as you see, this is not an easy answer or one that can be answered by just observing a swan. The Regal Swan