I understand certain processors are unsupported for Airplay support, but how do they go about enforcing this?
Certain processors are unsupported for AirPlay?
Not sure what you mean...
Please clarify your query
pac
I was told that Apple chose to restrict the AirPlay features in ML to newer models featuring Sandy Bridge processors (i5, i7, etc).
Where were you told this?
Where were you told this?
A simple hardware check?
...
It's not just the processors that were upgraded in the systems Apple is referring to. Many of them feature a newer WiFi adapter which enables some media streaming and dual streaming features. It's entirely possible some of those WiFi enhancements are required for the streaming to work.
To answer the OP's question, Apple has many ways of knowing which hardware it's OS is running on. It could merely check the driver version for the WiFi adapter or the Kernel's proc id functions.