Given family of grammars, determine if LR(k)
No, of course not.  If you want to prove that some formula is true for all $n$, it is in general not enough to show that it is true for $n=2$ and $n=3$.  That's just wishful thinking.

Consider, for instance, the claim that $n^2+n+41$ is prime for all $n$.  You can check it for $n=2$ (sure enough, $2^2+2+41=47$ is prime) and $n=3$ (yup, $3^2+3+41=53$ is prime), and it'll be true for both $n=2$ and $n=3$, but that doesn't mean it's true for all $n$.  In fact, it's not true for all $n$: at $n=41$, the corresponding value is not prime (it is divisible by 41).