It should be just a swipe with a cotton swab, if I recall correctly. They will probably swipe both your vaginal area and your rectum. Then they culture it and tell you if you are currently active with GBS or not. This is one of those gray areas where it's not certain that it's really necessary. Really depends on your feelings about antibiotics. If you test positive, they will want you in the hospital (or if you're homebirthing, they'll want to be at your home) as soon as possible. They'll want to give you antibiotics by IV at least two times, four hours apart. So, if you want to wait til the last minute to arrive at the hospital, you're kind of SOL. Less of a problem if you're homebirthing. I've known women who were GBS positive who homebirthed just fine and had the appropriate antibiotics. If you don't manage to get both doses before you deliver, which happens frequently, then your pediatrician may want to keep the baby at the hospital for 48 hours for observation or give the baby a dose of antibiotics directly. Both choices have their perils.
Good luck! Hope they don't find it. But, it's not a huge deal if they do.