Decidability of equality of CFL's
I am not sure there is any general characterization for equivalence, but the following papers by Hopcroft, and Hunt and Rosenkrantz resp. might be a good start:


John E. Hopcroft, On the equivalence and containment problems for context-free languages, Theory of Computing Systems 3(2):119-124,  doi:10.1007/BF01746517;
Harry B. Hunt, III and Daniel J. Rosenkrantz, On Equivalence and Containment Problems for Formal Languages, Journal of the ACM 24(3):387--396, 1977, doi:10.1145/322017.322020.


Hopcroft shows in particular that, if $M$ is regular, then $L(G)=M$ is decidable iff $M$ is bounded, i.e. there exist $n$ words $w_1,w_2,\ldots,w_n$ s.t. $M\subseteq w_1^\ast w_2^\ast\cdots w_n^\ast$.