All you have to do is open your phone's Settings app, tap the General section, tap where it says About, then scroll to where the serial number is listed. Once you've located your serial number, check to see if the fourth and fifth character matches one of the following, according to 9to5Mac:
Of course, it's worth noting that the only way to 100% confirm that your phone is eligible for the repair is to contact Apple's customer support of make an appointment at your local Apple store's Genius Bar.
Apple also requires that your phone be in "working order" to qualify for the repair, so that's a factor to consider as well. In any case, good luck dealing with this on top of all your stressful holiday shopping and travel plans.