I ask for lots of specifics about driving ability. Whose car do you drive? Do you get it serviced yourself? Where do you drive on a daily basis?
I really don't think most au pairs "lie" about their driving. Some may embellish a little. I find mostly they are judging themselves by the standards of their home country -- which may be significantly lower than ours.
One other thing I've done... I had a candidate from China who sounded "OK" on the driving, but great in other regards. I dug into her experience, it was somewhat limited. I told her that I had rematched with another au pair for driving reasons, and I really didn't want her to give up her job, etc. and fly half way around the world to come be my au pair, only to get sent home. So I told her I would extend her an offer to match as long as she understood that it was key to this job, and also if she agreed to spend significant time between matching and arrival (6 months) focusing on getting more experience.
FWIW, she didn't work out, and I had to send her home.
But -- I liked the theory of the stern warning about what would happen if I didn't agree with her that she was a suitable driver. I think it made the parting a bit more understandable to her.
But if driving really is key to your job -- I would stick with Swedes, German/Austrian, French... western Europeans. Their road system is very similar to ours, and in general they train their drivers better. Still, at some point, experience is key -- you can't "fix" a lack of experience with 5-6 driving lessons. Either they have it, or they don't.