Determine whether bit sequence is hash?
A SHA-256 hash is, until broken by cryptanalysis, indistinguishable from 256 bits of random noise. The only way to defeat this is by enumerating inputs until you find a matching hash.

If there isn't much entropy in the input (e.g., it's an English word, or it's a value that repeats), it will be relatively simple for an attacker to distinguish it from random noise and possibly even determine the data that was originally hashed.