I don't know a way to be absolutely sure they are good except to completely disasemble the rollers. If they feel tight they are still good.
I just replace mine every 15-20k miles and redo the water pump because I know they will fail.
Gunnerbuck has a very good thread on rebuilding them in the index.
I couldn't find his video but here's some of his wisdom.
We didn't see a picture of the metal on the magnet...were there broken pieces or just fuzz.
If it's chunks, I'd keep looking.
What year is your machine?