Which .x extension does my processor implement?
The Arm architecture includes a set of feature registers that report the features supported by the processor. For each new feature added by a .x extension, even the optional features, a field in these feature registers reports whether it is supported or not.
ID_AA64MMFR2_EL1.AT tells you whether there is support for the relaxed alignment requirements in Armv8.4-A. What you will not find is a field that reports that this processor is Armv8.1-A. Software reads the feature fields for the mandatory 8.1-A features, and if they all present, the processor is compliant with Armv8.1-A.
Arm Cortex-A processors
For the Cortex-A processors released at the time of writing, this table summarizes which .x extensions are supported by which processor.
Note: At the time of writing, no Armv8.3-A, or Armv8.4-A Cortex processors are available.