What website did you buy it from?
And there isn't a definitive way to find out exactly if it is authentic yellow tea or not. You'll just have to look at the leaves and see what the tea tastes like. If you've had yellow tea before you can compare the two. If not, you should have bought yellow tea from a well-respected vendor so you know you're getting what you wanted
Post some pictures of the leaves when you receive it. Maybe the packaging too.