Frustratingly, every company that attends these job fairs isn't always hiring. Some are simply out to promote their company, troll for qualified applicants and save the best resumes for the time when a position opens up. But if you're lucky enough to engage a recruiter in a conversation about a specific open position, don't let it go to waste.
One subtle, yet effective way to show what you're made of is to ask about the biggest challenges of the job. When you ask about challenges, it accomplishes several things at once. First of all, it's not a question that the recruiter has heard a thousand times, so it snaps the recruiter out of his or her routine. Secondly, it shows that you're someone who is doesn't shy away from hard work. Thirdly, this question provides an excellent segue to a conversation about how you and your specific skill set are more than up to the challenge.