Use minicom and set the serial device as /dev/tty.baseband, then enter the command:
AT+XSIMSTATE=1
Compare the result with the following:
+XSIM: SS
+XLOCK: "PN",X1,X2,"PU",X1,X2,"PP",X1,X2,"PC",X1,X2,"PS",X 1,X2
(note: on iphone the only lock present is the PN lock)
SS:
0=No SIM Inserted
2=SIM Ready
6=SIM Busy
X1:
0=Unlockable(i assume without attempt counter)
1=Unlockable
2=Signature check failed
3=Attempt Limit Reached
4=Unlocked and Lockable
5=Unlocked
X2:
0=SIM Valid
1=Unknown(this gets set for PN when unlock is attempted)
4=SIM Not Valid
These values weren't found in any manual, and were all determined either empirically or by reversing the firmware.