How can I determine the start non-terminal of a CFG?
By definition a grammar is a tuple $(N,T,P,S)$ where $N$ is the set of non-terminals, $T$ is the set of terminals, $P$ is the set of productions, and $S \in N$ is the start symbol.

Therefore if you have a grammar you already have $S$ (and if you don't know $S$ then you don't have a grammar).