
First of all you get them hip-scored once they are 12 months old. This involves a general anaesthetic and x-rays taken, which are sent to the Kennel Club to be officially scored. The breed average score is 19, so any score higher than that, and you can forget using them. They will also need to be cleared for haemophilia.
A good idea, to find out if they are good enough to use at stud, is showing. This way you'll get an honest opinion as to whether or not their use would be beneficial to the breed as a whole. If not - well, there are already umpteen poorly bred GSDs out there, and plenty in rescue all the time - so unless they are very close to the breed standard you won't get any bitch owners wanting to use them.