Decryption of ElGamal ciphertext

If someone says that the decryption of an ElGamal ciphertext is "x", how can I be sure that the real value is "x" and not "y", even if he is the owner of the respective private key?

If DDH is tractable and they assert that the cryptogram $$(n,c)$$ is of the form $$(g^r,xa^r)$$ then you can check $$\mathrm{DDH}(g,a,n,c/x)$$.