Butter can go bad. The oils will go rancid if exposed to too much light and heat for too long. This accelerates the process of oxidation, which happens even if you keep the butter in the fridge. Even in the fridge your butter will eventually go bad.
Personally I follow the dates on all packages. I do so with an awareness that they usually tend to be conservative, which keeps me comfortable. Rancid butter in a taste test won't kill you, though. It'll just taste bad. So you should be able to taste this butter to see if it is bad.
While I recommend following the dates on packaging, here are signs that your butter has actually gone bad:
- melting (which will happen outside of the fridge, you probably wouldn't see it)
- sour smell
- sour and unusual taste