@DancingMirren - Honestly, you can do everything 'right' and still end up with a dog that has problems and there is no way to be 100% sure - even the smallest external influence can make a dog nervous or give it 'issues.' It's part of the reason why breeders who breed their dog 'just the once because she's lovely' wind me up so much and I find them so thoroughly awful.
There are things to look for when you meet a litter - i.e. does the bitch look nervous, do the puppies play well, how does mum discipline them, how does the breeder interact with the pups etc.
I do all I can to ensure even-tempered pups but, as with the fact I've not yet lost a puppy, part of my success is down to luck. You can plan and plan and plan, but nature will do what nature wants.
When I pick a sire, it's almost always one I have met in person - some breeders do it all online, but I like to meet the dog first (usually at shows) and make a judgment call. I also match owners to the puppies and whilst I'll take the 'sex' preference into account, if there isn't a puppy of their chosen sex with the right temperament I will tell them that and either put them back on my wait list or offer them one of the dogs of the opposite sex that does have the right temperament. I make that 100% clear when I speak to prospective buyers.
Unfortunately, I've found resource guarding and other 'issues' (including aggression) to be on the rise - but not just Goldies. I've also seen it on things like Cockers, over the last 10+ years, Labs, Boxers etc. All the breeds that typically are 'family friendly' and I can't help but wonder how much of that is bad breeding...