It is a little late to have the swans sexed as this will be a disturbance since the birds will need to taken to a veterinarian experienced in sexing a bird. Even then, nothing is 100% foolproof except DNA sequencing in which a feather or blood sample is submitted to a licensed laboratory.

Egg candling can be conducted by someone who knows how to do the procedure. If done incorrectly, the egg can be disturbed in such a way that the embryo can become detached, basically killing it. Are both birds laying the eggs? You may just have to wait and see (35-40 days) to see if the eggs are fertile. If you get cygnets, then obviously, the swans are male and female. If not, then they both are possibly female. Infertile eggs do not necessarily mean a swan pair of same gender as age, environment or climate can affect if eggs reach fruition. The Regal Swan