Hypothetical question really, but I've always wondered how you could find out what grade of Stainless Steel you were about to weld i.e. was it 304 or 304L or 316 etc. so you would know what filler rod to use...
The simplist one is sulfuric acid, 304 goes a dusty green colour, 316 goes a browny colour.
Hydrochloric acid can be used aswell, that reacts with more grades, but is not quite so easy, less distinguished results and takes longer.
At work I'd send a sample to the Met lab, and they'd run a few properties tests and take a look under the electron microscope. Shame they wont do personal stuff really.