First, WELCOME to YT!! You will find so much support here! If you truly want to learn about Yorkies and LOVE the breed, then you've found the right place.
Glad you're here.
As far as your question, what concerns me is I see "the cart before the horse". In other words - you're thinking about breeding - which is INCREDIBLY difficult, complicated, and risky. But, it doesn't appear that you're familiar with owning a puppy, in general, yet - since you're not sure what to ask a vet, in terms of what defines a health check. So, on that front, I'd encourage you to head to the breeding section and check out what
even the most experienced breeders go through with birthing litters, bc it can be heartbreaking and eyeopening.
For the check, the vet should know what to do for a new puppy wellness check - but he/she should do a full physical - including vitals. At this point, checking knees for LP won't likely show much - blood work wouldn't likely show much either (unless something is wrong already, or a suspicion), so that's kind of up to you. But, it is a very good idea to get her checked over for any obvious red flags.