Vent checking or probing is the least reliable method unless the swans are 1 year older or more. The male genitalia will not be developed prior to that age. And, just as you described, you may think the swan is one gender and later, discover that in fact it is opposite what you think. In probing, you will check to see if the male genitals are present.

The most reliable is to sex the birds using a plucked feather with blood attached. Yes, there is a slight discomfort, similar to you pulling a hair from your head, but you only need one feather from the chest with a small amount of blood. Submit the feather to a licensed laboratory for DNA sequencing. Within two weeks, you will have a reliable answer. If you are checking several birds, ensure that you have a method to identify the bird from which the feather was procured and use that identification on the package label. Each birds' feather should be placed in a separate container and labeled with the proper identification for the lab. The Regal Swan