The IV is XOR'ed with the plaintext before encrypting the first block. You can get the input of the block cipher by decrypting the first block of ciphertext. After that it is a simple operation to get the IV, of course, considering you know the plaintext and - hopefully - the mathematical properties of XOR.

The IV is XOR'ed with the plaintext before encrypting the first block. You can get the input of the block cipher by decrypting the first block of ciphertext using the block cipher directly. 

After that it is a simple operation to get the IV, of course, considering you know the plaintext and - hopefully - the mathematical properties of XOR.