Language consisting of all Turing machine encodings
The complexity of $A$ depends on the encoding used for Turing machines. It is easy to come up with an encoding in which every string encodes some Turing machine (there are lots of ways). In contrast, it is easy to come up with artificially "hard" encodings, say the $k$th Turing machine being encoded by $1^kb$, where $b=1$ iff the $k$th Turing machine halts on the empty input; under this encoding, $A$ is not decidable. Nevertheless, it seems intuitively clear that any reasonable encoding makes $A$ decidable, though it's hard to say anything more without knowing the exact encoding.