magnetic chip detectors
Debris on the MCD if the engine is new can be build debris as the engine is assembled and screws and nuts tightened down, so is normal.
Metal on the MCD after initial build debris has to be assessed as to quantity and size.
A nice big chip from a gear tooth is a problem, a few slithers can be carried on for a 100 hours and be assessed again.
Big companies would have a department which analyse the fines from MCD after each daily check and can predict when the engine would had to be changed as a thrust bearing or gear wheel is shedding metal.
Down the line the engineer would not have the lab or ability to analyse the type of metal on the magnet by its chemical make up, so can only guess.
Metal on MCD is covered in Maintenance manuals in ATA chapters 72 or79