Every time I play with a new 3G or 4G stick, I need to spend some time searching for AT commands to be used for basic tests. Today, at last, I decided to spend 10 minutes storing in a place easy to access (here) the latest available, official 3GPP Technical Specification listing standardized AT commands Technical Specification.
When using this document, do not forget that implementation of the whole AT command set is not mandatory. On the other hand, a manufacturer may choose to add its own commands (which, in practice, is always the case).