How can I determine the start non-terminal of a CFG? Suppose I have a grammar such that there exist $n$ production rules which contain only terminal symbols, and none of these rules produce the same terminal (disjoint).

$A ::= x|y|z$
$B ::= a|b|c$
...
$N ::= l|m|k$

Further suppose that I cannot simply merge these production rules together as they are used in different parts of the grammar.
How can I hence determine or rather choose, the start non-terminal $S$, for purposes of $LL(1)$ parsing?
Thanks
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).