A cashier *asking* you if you want to pay it forward goes against the basic spirit / norm of paying it forward (the gift economy system more broadly). The only thing they could / should do is tell you that your meal has been paid for and leave it at that -- only if you actually ask what you can do, should they offer the info of how you can contribute and even then, it's *not* about paying for the next person as much as what do you feel like giving (which could be even a few cents or nothing) in exchange.