I would like to be able to determine if third party calculated shipping rates are enabled on a store from my app I've developed. How can I test the truth of this, other than trying and failing creating a carrierService?
Go to your dev/test store. In Shopify Shipping & delivery settings. In the Carrier accounts section, if you have the ability to connect a carrier account to your dev store, 3PCC shipping is enabled. If you see a delivery truck icon and are prompted to activate, the feature is not enabled.