Identifying 303 stainless vs 304

I do contract machine work. No matter how many times I tell my people to positively identify materials before restocking them, sometimes there is a problem. We use both 303 and 304 round bar. Our suppliers do not have a standardized color code. Sometimes a customer will require 304. I have some stock that is mixed up. Is there an inexpensive test I can do to tell the difference between the two and get back on the right track?

Inexpensive is relative, but for my pocketbook, the answer is NO.
A suggestion: Buy two different colors of paint stick (it is like a color crayon and in several colors) and color both ends of the rod in your color scheme for each type. Mandate that the machinist color the cut end of the rod before returning it to stock. It is cheap and fast and as foolproof as your machinists are.