Drain the oil in a clean bucket and run a magnet thru it, a little "fuzz" is common, chunks and pieces are not good
If this has not been done in a while and if you have NO chunks or water intrusion, re-fill and run (w/new gaskets on plugs) for a baseline...
Drop the oil again and see what color it is, milky = water intrusion, ferrous glitter can be a bearing going out or pinion wear... Non ferrous contaminats could be a bearing race, or housing material...
If in doubt, bring it to your favorite outboard shop...