All you could do in terms of testing is get a baseline lyme titer count, which would just get you a number that you would equate to normal for your horse. It's really not worth squat. If your horse gets Lyme, the symptoms are obvious enough, so don't waste your money up front.
That being said, the treatment is very pricey. I spent $800 last year on Doxycycine. Prevention is the way to go. Pick up a few tubes of Equispot. It's a 3 pack of orange tubes that you apply to the back, poll and legs once every two weeks. It seems to work. At $14 for 3 tubes, it's worth the effort.
If your horse does contract Lyme, quick treatment is key. It's as simple as knowing your horse. If you suddenly see him acting cranky, ouchy, doesn't want to be touched or brushed, take his temperature. Anything over 101 suggests a possible infection. At that point, have your vet do a western blot test to check the titer levels. The treatment gets very quick results. While you usually have to run an entire 21 day course of doxy, the horse usually perks up and feels better within a day or two.
Just keep an eye out for those ticks. Don't forget weird spots like under his chin and under his tail. Good luck! Hope I get lucky this year too.
You just have to see your distance...you don't have to like it.