Script that queries pair of BIG-IP LTM to detect which one is Active

using iControl REST, you could for example query the following path with curl:

curl -sk -u admin:mypass https://my-bigip/mgmt/tm/cm/device | python -mjson.tool

This will return a json formatted list of all devices in your device group. One of the fields for each device is "failoverState", that will either return active or standby.