Four questions, in order
1. Letters or numbers? Numbers in pairs, all between one and five, means a square: Polybius or tap code. Long numbers mean a book cipher or a computer.
2. How many different symbols? Two means Bacon or binary. Five means ADFGX. Twenty-six means an ordinary alphabet cipher. More than twenty-six means homophonic.
3. Do the letter counts look like English? If E, T and A are still the commonest, the letters have been moved, not replaced: it is a transposition. If the counts are flat, something is fighting frequency analysis.
4. Are the word lengths normal? Groups of four or five mean the sender hid the word breaks on purpose.
The most useful skill on this site
Every challenge in the capture the flag section starts here, and so does every real message anyone ever intercepted. Nobody hands you a cipher with a label on it.
And the last example is the one worth remembering: sometimes the answer is that there is no message. Pi looks exactly like a code and is not one. A code-breaker who cannot say this is nothing will spend years on it, and some have.
Try the challenges Frequency analysis A cipher identifier toolRead more elsewhere
Longer, harder and written for grown-ups. Worth a look when this page runs out.