Solution: Go to your Admin -> System -> Configuration -> Shipping Methods, right click and Inspect Element on the actual service, such as "Next Day Air" and the API name will be the carrier plus whatever is in the Value property of the service you right clicked.
Examples:
USPS_0_FCLE - First Class Mail Large Envelope
USPS_1 - Priority Mail
USPS_INT_1 - Priority Mail Express International