This is just standard SNMP, you can easily find this by doing an snmpwalk on the PIX. Mine shows the following:
interfaces.ifTable.ifEntry.ifAdminStatus.1 = up(1)
interfaces.ifTable.ifEntry.ifAdminStatus.2 = up(1)
interfaces.ifTable.ifEntry.ifOperStatus.1 = up(1)
interfaces.ifTable.ifEntry.ifOperStatus.2 = up(1)
or if you want the numbers:
.1.3.6.1.2.1.2.2.1.7.1 = up(1)
.1.3.6.1.2.1.2.2.1.7.2 = up(1)
.1.3.6.1.2.1.2.2.1.8.1 = up(1)
.1.3.6.1.2.1.2.2.1.8.2 = up(1)
Interface 1 is outside, interface 2 is inside.