The FAA have developed a free for use and publicly available website and API for both ADS-B and RAIM (Receiver Autonomous Integrity Monitoring) pre-flight predictions. The ADS-B half of their solution is called SAPT (Service Availability Prediction Tool).
SAPT acts as a free reference implementation, however it is not provided under license with guaranteed high availability and 24x7 support. The FAA do permit third parties to provide ADS-B prediction tools and services for use by the world-wide aviation industry. NAVBLUE already have a portfolio of commercial RAIM prediction solutions and N-Prediction is the latest addition.
In case of SAPT unavailability, NOTAMs should be issued by FAA, and published in FAA OIS page (https://www.fly.faa.gov/ois/)