This is a great link to bookmark. It will show what your vet can tell from a simple blood chemistry panel:
http://www.bichonfriseusa.com/caninebloodwork.htm
CBC's are very inexpensive. Prices vary depending on where you live, but under $50.
Most vets recommend getting one done annually until a dog reaches the senior years at age seven. After that, twice a year is recommended. It is a great tool to troubleshoot little problems before they become serious issues.
In addition to CBC's, all Yorkies should have a bile acids test done preferably before they even come home, but at least before their spay or neuter. Since Yorkies are 36 times more likely to have a liver shunt than all other breeds combined, it's essential to have this test. Again, this is a very inexpensive test, between $40-$75.
As they age, it's good to get a thyroid panel done, too.