Swan upping

Here is the swan family that I blipped earlier in the year, unsure whether their parents had moved them from Seafield Pond to the safety of the River Bielwater. As you can see the cygnets are growing fast and there are still four of them. I now think that they cannot be the family on the pond as an adult swan appeared just yesterday to have returned to Seafield Pond and is hanging about at the very spot where I saw swans nesting earlier in the year. I assume this is either the male or female swan hoping for another partner. How do you work out their gender when you cannot compare sizes nor get close enough to check out their anatomical features ?