How to double the amount of xp that you get?

I'm making a mod that lets the player level up using xp. Whenever you level up, you get a reward or two. To get to max level, you need a lot of xp. Instead of changing the xp required so that it's less, I want one of the rewards you get to double the xp that you get from then on. I have the variables and everything set up, I just need to figure out how to tell when a player is getting xp and how much they just got so that I can multiply that number by two.

Is this possible?