You be polite by agreeing to the call before you ask for the agenda. I'd also give the reason why you want to know, as being prepared for the call is good for both of you. So I'd reply with something like the following:
Sure, I'm available to talk at X time. So I can be prepared, what's the agenda for the call?
If "agenda" is too formal, then say something like "topic" or "what will we be discussing" or whatever sounds natural to you. They aren't likely to care about the specific terms you use.
I've never used a recruiter specifically, but I've used this sort of language for many, many calls and meetings. Nobody has ever been offended, and it almost always makes the call more productive for everyone involved. It's only perceived as rude when it's not clear why you're asking, and so they assume it's because you're trying to figure out if it's worth your time to speak with them or not. Make it clear you're asking for a different reason, and you'll be fine.
If the recruiter then springs something on you that you need to think about, feel free to tell them you need some time to consider it and will get back to them by X day/time with an answer. Again, I do this all the time, and it's never been an issue if I clearly state when I'll get back to them.