It seems that the information may be in Intent extra with the key "simSlot".
public void onReceive(Context context, Intent intent) {
...
int simSlot = intent.getIntExtra("simSlot", -1);
...
}
I also could not find information about this in my admittedly brief search, so I'm not sure how universal it is or in which version of Android it could be introduced. I found it by dropping all the add-ons in the supplied intent in the receiver on my device.