What can I do to prove a gift was a gift after a break up?

He will not win if his claim is based on the facts you describe above, but it won't be.  He will claim that the money he paid for the Keurig was a loan, or that it was his and he was just keeping it at your apartment because he spent a lot of time there while you were together, or some other claim other than that the Keurig wa a gift.  There is no guaranty that a court or jury won't believe his story rather than yours, but if you can show that you received the Keuring at Christmas, that you gave him a gift, that the gifts he gave you other than the Keurig (if any) were not commensurate with what you gave  him or with previous gifts on other occasions, I think it is likely that you will be believed.  Obviously, it would be even more helpful if you had a witness, somebody who say him give it to you, or whom he told that he was giving you a coffeemaker for Christmas.