The only good way to tell if a coconut has gone bad and is spoiled is to cut the coconut open and look inside the coconut.
If the coconut smells bad or looks bad inside then it's spoiled and should be thrown out.
A fresh good coconut should be hard and you should hear the coconut milk splashing around inside the coconut when you shake it.
But I would just cut the coconut open and look and smell.
However if there are any cracks or bruises on the outside of the coconut then the coconut is likely bad or going bad.
But cutting the coconut open and looking at it and smelling it will let you know for sure whether or not the coconut is still good or if the coconut has gone bad.
A fresh coconut that has not been cut open should last easily for 4 months at room temperature and if you've had the coconut longer than 4 months then it likely is bad.