Frontline isn't a repellant. Rather, it kills fleas and ticks that attempt to feed on your dog. With respect to ticks, it kills them before they fully embed and have time enough to transmit the bacterium that causes Lyme. While you still may pull ticks off your dog, they likely will be dead and desiccated. Assuming that your dog is inoculated, that you're using Frontline regularly and as directed, and that any ticks you find are dead, you needn't test for Lyme every time you do find one on your dog. Of course, if your dog starts acting logy and you suspect that it's ill, your vet may want to test for Lyme.
Keep using the Frontline and have your dog tested and inoculated for Lyme annually. When you're going to be in the woods, or in grass or leaf litter where ticks are likely to be found, it's a good idea to spray a small amount of insect repellant on your dog's back and coat (don't overdo it, and not on the face and around the eyes obviously. I put some on my hand and rub it on my dogs' coat).
When your done walking your dog, roll its coat with a sticky-tape lint remover. Ticks like to attach around the dog's head and neck, where the dog is unable to get to them with its mouth. If you use a lint roller, you often can get the ticks - even deer ticks that are hard to see - as they're attempting to migrate across your dog's coat toward its head.
I use a roller on me too -- I've just gone through a treatment for Lyme (my dogs have never had it) and I have no idea how I got it. I don't remember being bitten, or finding ticks on me, and I never had any telltale rash. One of my dogs certainly could have brought a tick into the house, or I could have been bitten while hiking or riding my mountain bike in the woods. If you play outdoors where I live, chances are that you will be exposed to Lyme.
If you want to try an experiment, drag an old white towel through tall grass or leaf litter where you suspect a tick infestation. The number of ticks you pick up can be pretty startling.