Whether a given language is context free or not? Is it decidable or undecidable?
Undecidable. Given language could be anything... NOT RE or RE but not REC or REC or Regular....anything.
If this problem had been Decidable then we could decide many problems which are already proven to be Undecidable. Like consider this "Given a $TM$ $M$, Whether $L(M)$ is CFL or Not??"...We know, by Rice's theorem, that It is Undecidable.
The problem statement "Given a $TM$ $M$, Whether $L(M)$ is CFL or Not??" is equivalent to saying "Given a RE language $L$, Decide whether $L$ is CFL or Not??" ..Which is Already proven to be Undecidable.
Hence, "Whether a given language is context free or not? " is Undecidable.