Verification of encryption for ElGamal cryptosystem
Actually, an ElGamal ciphertext $(a,b)$ is "valid" iff $(a,b) \in G^2$,  because for any message $m\in G$ and any $\mathsf{pk}\in G$, the encryption of $m$ will be in $G^2$, because for any $r$, $(g^r,\mathsf{pk}^r\cdot m)\in G^2$, and any $(a,b)$ chosen in $G^2$ can be correctly decrypted by any secret key by computing $m=\frac{b}{a^\mathsf{sk}}$. The proof of knowledge of the random coin $r$ is sufficient to show that you know how the ciphertext was produced. 

I do not know if that answers your problem.