At the clinic I worked, we tested for lyme with the yearly heartworm test using the 3DX Idexx Snaptest and now 4DX Idexx snaptest. The snaptest detects C6 antibodies which are from natural infections and will not cross react with vaccination caused antibodies, unlike IFA and western blot tests. So you can use this test on vaccinated dogs.
With the dog that tests postive with no obvious clinical signs, I recommend an UA and urine protein/creatinine test to check for protein in the urine to check for kidney involvement. If positive for that recommend treatment with doxycycline. The others, I discuss with owners doing a quantative c6 test to get a look at actual antibody level to determine need for treatment(<30 don't treat and monitor , > 30treat and retest in 6 months) vs treatment without the quant C6. Will also talk about tick preventatives and daily check over of the pups to see if their is anything we can do to boost reexposure prevention.