A quick search turns up a quote of Shakespeare from Romeo & Julia. It is often a good idea to simply search the internet, even for (smallish) hexadecimal values or values encoded as base 64.
In general it can be expected that decryption has succeeded if the attacker gets English text without invalid words. Words are usually not encrypted separately, so if they are correct the likelyhood that the decryption succeeded is very high.
To check that the plaintext is correct it is required that the ciphertext size is at least somewhat larger than the key size. If it is not then it is impossible to distinguish between the correct plaintext and other candidates. OTP, where the key size is the same size as the plaintext/ciphertext, is a good example of the latter; each possible sentence has the same probability.
In your case the key is very small (the number 9), so the key must be correct to get this plaintext. The only way that another key would also be valid is when the creator of the key / ciphertext deliberately allowed two different outcomes. This is probably not possible for a Caesar cipher over a sufficiently sized sentence though.
Note that it's common for these kind of puzzles to have a plaintext that is just slightly wrong. I'm presuming some professor or assistent is silently sniggling somewhere...