Using letter frequency

In normal English, E, T, A and O are the most common letters. Counting the letters in a cipher can tell you what kind it is. If the pattern looks like English but shifted, it is probably a Caesar cipher. If the letters are fairly even, it may be Vigenère or another cipher that uses a key.

The index of coincidence puts a number on this. English text scores about 0.067. Random letters score about 0.038. Simple substitution ciphers keep the English score, while Vigenère moves it toward random.