For freemill gold, easiest test is to just crush it and pan it. Invisible gold, or sulfide gold would be hard to find without proper assay or at least nicely callibrated XRF in Geochemical mode.
Iron... depends. Crush and pan can be also a way to go, concentrating heavier pieces with iron oxides. Some iron oxides can be magnetic, if they contain some portion of Fe3O4 (magnetite), so this can be somewhat useful.