how distinguish the difference between 304 s.s. & 316L s.s.?

Answer 1

If you are checking in the field, take a small magnet with you. The 304 will attract the magnet. 316 will not cause a magnet to stick.

Answer 2

In general, the higher the number sequence, the less magnetic signature the material will have. This means 400SS is less magnetic than 300SS, etc.

For most purposes, instruments aren't sensitive enough to determine this - mostly because manufacturing processes such as cold-forming and hot-forming produce variance in the signature as well.

304: <0.08% carbon. 8-11% Nickel. 18-20% Chromium ... which basically gives it a slightly higher tensile strength (75 ksi).

316L: <0.035% carbon. 10-15% Nickel. 16-18% Chromium. 2-3% Molybdenum (there's none in 304).