More often than not, your speaker will say yes when you ask this first time. But the next thing you need to do is the Double Confirm, which means telling your speaker that you're going to confirm them for your event — and even mentioning a certain time slot.
You want to hear them say, "Okay, that sounds good." And if they don't, be sure to take a step back and ask them to let you know when they can confirm.
Once they say that they're in, you should run through a list of what they're committing to, right there on the phone.