For Boeing, RNP Requirements are detailed in FCTM, and for Airbus in Instructor support section on GPS Primary description, although again mentioned in the training materials on approaches and NAV ACCURACY.
The whole RNP subject is also detailed in Operations Manual B for most companies, covers en-route, terminal and approach RNP requirements.
General rule is " Unable Reqd Nav Perf" or " Nav Accuracy Low" you are stuffed.