The way to do this is on a case-by-case basis. Attempts to label things in advance of investigation are overly ambitious.
Any attempt to utilize isochron dates is likely to fail, since these don't take the flood into account. Naturally occurring water can dissolve the ingredients, so there's no way to easily determine initial conditions even if one were inclined to do so.
Fossils are advantageous to the investigator, because they leave clues as to whether a rock would be most likely to have been formed before, during, or after the flood.
Due to curiosity, I'm most interested in the pre-flood world. It is fascinating, and it's also one of the most difficult subjects to research. So rather than view an example which seems as if it couldn't be formed after or during the flood as a problem, I view it as a potential pre-flood specimen that may contain new information. But I've learned to be skeptical of claims about what the flood could not do.