Finding a Caesar cipher key given encrypted message and part of decrypted message? How can I find the Caesar cipher key if I have an encrypted message and part of the decrypted message? For example: I know that the message ends with a constant set of words, but the length of the key is not known.

I have tried using the Kasiski test to check for repeating strings in the encrypted message, but the message i have is a very small one (around 200 characters) so i don't see any useful patterns that might help me narrow down on the length on the key.
For the Caesar cipher there are only 25 possible keys (and the key $k=0$ which degenerates into the identity function, ciphertext = plaintext).

Just try all possible shifts one by one starting from $k=1$ to $k=25$ (this constitutes a brute force attack). After each shift you need to validate that it results in an intelligible plain text by decryption of the ciphertext. For the Caesar cipher brute force attack is efficient.

Now if you have plaintext and corresponding ciphertext pair work become more easy you don't need to do brute force attack. As in generic Caesar cipher encryption/decryption is defined as

$$C = (P + K) \bmod 26$$

$$P = (C - K) \bmod 26$$

Then from above $K = C - P$ (for each corresponding ciphertext / plaintext character).

For example if the plaintext is "abc" and the ciphertext is "def". Then teh 'd' zero-based index is $3$ within the alphabet. And the and 'a' location is $0$ then $K=3-0=3$