A question on decidability
Let $L=\{\langle M \rangle \mid M \text{ is a TM such }L(M)\in R\}$, where $\langle M\rangle$ is the encoding of a TM $M$, and $R$ is the set of all decidable languages. This is the language in question.
Notice that $R\neq \emptyset$ and also since there are languages not in $R$ (like the halting problem), then $R\neq RE$. Simply put, $R$ is not trivial (obviously).
Apply Rice's theorem on the property $"R"$, to directly get that $L$ is not decidable.