If there are around 26 characters, then read on.
If there are more than 26 characters it is likely to be a code or nomenclator of some sort or a homophonic substitution cipher. If there are 5 or 6 it is probably a polybius square cipher of some sort, or it may be ADFGX or ADFGVX.
How many different characters are there? If there are only 2 different symbols, it is likely the cipher is Baconian. Very short ciphers may be unbreakable if their length is less than the unicity distance of the cipher used to encipher them. If all you have is 20 characters there is not much you can do. All of the methods explained here need quite a bit of ciphertext, 1000 or more characters is ideal. Some of the fingerprints are very faint though. Given that there are so many different ciphers, how can we expect to identify a piece of ciphertext? Different ciphers leave different 'fingerprints' on the ciphertext which we can use. Examples include bifid, trifid, ADFGVX, Straddle checkerboard etc.