Use TelephonyManager.
http://developer.android.com/reference/android/telephony/TelephonyManager.html
As Falmarri notes, you will want to use getPhoneType FIRST of all, to see if you are even dealing with a GSM phone. If you are, then you can also get the SIM state.
TelephonyManager telMgr = (TelephonyManager) getSystemService(Context.TELEPHONY_SERVICE);
int simState = telMgr.getSimState();
switch (simState) {
case TelephonyManager.SIM_STATE_ABSENT:
// do something
break;
case TelephonyManager.SIM_STATE_NETWORK_LOCKED:
// do something
break;
case TelephonyManager.SIM_STATE_PIN_REQUIRED:
// do something
break;
case TelephonyManager.SIM_STATE_PUK_REQUIRED:
// do something
break;
case TelephonyManager.SIM_STATE_READY:
// do something
break;
case TelephonyManager.SIM_STATE_UNKNOWN:
// do something
break;
}
EDIT:
Starting at API 26 (Android O Preview) you can query the SimState for individual sim slots by using getSimState(int slotIndex)
ie:
int simStateMain = telMgr.getSimState(0);
int simStateSecond = telMgr.getSimState(1);
official documentation
If you're developing with and older api, you can use TelephonyManager's
String getDeviceId (int slotIndex)
//returns null if device ID is not available. ie. query slotIndex 1 in a single sim device
int devIdSecond = telMgr.getDeviceId(1);
//if(devIdSecond == null)
// no second sim slot available
which was added in API 23 - docs here
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