Vent sexing isn't always that easy for the inexperienced.
I've attempted it and even teamed up with a fellow goose breeder had problems.
I can sex rabbits with accuracy all day long but with geese it can be quite difficult.
Finding the vent thru all that thick down is a feat in itself
plus trying to invert the vent of a screaming, flapping large youngest or adult
can be trying.
With goslings I always feel like I'm going to squash them.
As much as I've handled animal over the years I think some people just don't
have a knack for vent sexing and so when in question DNA testing is the answer. www.dnasexing.com is another one.
It may not be the same for poultry, but DNA testing does much more than just determining the sex. I know for goats and dogs, they can send you additional information about other genetic factors. For example, for dogs - the DNA testing can tell you if the dog will be predispositioned to have joint problems, etc.
It costs about 20 or 25 dollars per bird. Get a bird talk magazine and look in the classified section. You order the kit and they will send it to you. To do it you take fingernail clippers and clip off the end of their toenail. Just enough to make it bleed and collect a couple of drops of the blood in whatever collection container. Send it back and later you will recieve a certificate through the mail telling you the sex of that bird.
You can also just pluck a couple feathers and send them on their way. You don't even have to wait for the kit. Just go to their site, print out the form, pull the feathers and send them on their way. I have used these guys a couple times... http://www.avianbiotech.com/ and have had no issues.
There are DNA tests available that have been used to demonstrate if the offspring are indeed those of the parents. I have not seen it advertised and would think its very expensive. Previously used in a legal case to prove offspring did not come from the parent birds they were reported to be from!