Is it spoiled?

How do I know for sure if the wine is bad? It all tastes sorta bad to me, as if I can taste the fermented grapes themselves. I had a glass of the Yellow Tail tonight and for some reason it tasted a bit off to me, but how do I know for sure? I drank it anyway, figuring this cold is messing with me and factoring in the fact that I am not very good at distinguishing tastes in wine. Ah well.