Given an encrypted file, what are possible ways to find its encryption algorithm? Given an encrypted file, what are possible ways to find its encryption algorithm?

Heuristic ways are okay as well.
One of the important properties of a modern cryptographic algorithm is that it resist distinguishing attacks. Distinguishing attacks are any way to tell the encrypted data apart from random data. So if you can tell what the algorithm used was simply by looking at the ciphertext the algorithm is vulnerable to this attack.

Unless there is a header or such that tells you the algorithm and mode used, or a weak cipher was used, determining the algorithm used from the ciphertext is impossible.