If I have the key, the ciphertext under AES-CBC and the plaintext, then how can I guess the initialisation vector?
CBC mode decryption for the first block is defined as:
$$P_0 = IV \oplus D_k( C_0 )$$
where $P_0$ is the first plaintext block, $C_0$ is the first ciphertext block, and $D_k$ is the decryption by the block cipher using the key $k$.
This can be rearranged as:
$$IV = P_0 \oplus D_k( C_0 )$$
which allows you to reconstruct the IV, if you know the key, the plaintext and the ciphertext.