There's really no way to get a hard version number. Modern IOS releases can be tri-lingual depending on the config. If you are using 11.3 or later, and you have at least one SNMP community string configured, your router can be queried with SNMPv1 and SNMPv2c. Starting in 12.0(3)T, we added support for SNMPv3. For that, you will need an SNMP user and an SNMP group configured.