Boat: Sold Sigma 33 OOD some time ago, will be chartering in Turkey really soon
You cannot tell the difference easily. Looks and feels exactly the same.
A chemical analysis is an (expensive) possibility.
NDT companies could use a portable analyzer like a Texas Nuclear.
There is a fluid (ask your local inspection company)that will indicate the difference; I guess it looks for titanium which is added to 304 to make it 316 and seawater resistant.