The confirmation email matters, but it's not bulletproof on its own - I've seen places try to weasel out of stuff anyway. The real move is to log into your booking account (wherever you made the reservation) and check what it says there about breakfast. Most booking sites show you exactly which amenities are tied to your specific room type and dates, so you can verify it's actually part of your reservation before you even contact the hotel. Take a screenshot of that page just to be safe.
After you've confirmed it online, then reach out to the hotel directly - but here's where the other answers are spot on about having your confirmation number ready. Don't just ask "is breakfast included?" Ask them to confirm it's included for your specific dates and room category. Get them to put it in writing via email if you can, even if it's just a reply saying "yes, complimentary breakfast is included with your booking." This way you've got a paper trail if checkout somehow goes sideways.
If the booking site or confirmation email mentions breakfast is only for certain room types, double-check that you booked the right category. Sometimes the cheaper option doesn't include it, and it's easy to miss that detail when booking fast. Most legit hotels won't surprise you with a charge if breakfast was clearly listed when you booked, but having that documentation means you've got leverage if they try.