you're right to be worried - there's way too many people breeding yorkies right now
but I think the basics apply - you can read the Breeder section to learn more about the health issues - but if you do find a puppy you're interested in - try to spend a little time with him/her before deciding.
I don't think there's any way to predict the health - but you can take steps to make sure the Breeder is breeding nice healthy dogs - see the parents - the enviornment - and vet records to start with.
Other than that - maybe someone more experienced can help you - I've only done this 2 times...I feel I was very lucky with Chanel and Cheri ....especially Chanel considering where we got her.