Cerebro is the mutant detector
While Cerebro, and now Cerebra, lack the ability to act as precisely as a Dragon Ball Z scouter by giving a specific rating to a mutant's power level, they have, since 1975, had the ability to gauge a mutant's relative power to determine its potential threat level.
- Cerebro has on various occasions been able to make its users aware if a mutant possessed significantly greater abilities than the average mutant.
Since Xavier says Krakoa defies classification, Cerebro must give some simplified scale by which a mutant's abilities can be rated.
Krakoa was originally a very small island in the Pacific Ocean that was located close to where some nuclear bombing tests were done. The radiation somehow turned the island's ecosystem into a hive-mind entity.