film is the primary tool they use to evaluate players. you would be surprised at how often they (not just rivals but all services) have diffiuclty in getting film from high schools, players, coaches, etc. etc. Even the measureables come from most anyone that will talk with them, including girl friends...
AND, it is not a huge staff at any of these services to do the evaluations.
Why fans should take with a grain of salt. I've been to Rivals headquarters many times so I am familiar with how they operate.