How do I find out if the metaltic rock I have is platinum or what it is? .
You can test for specific gravity (platinum is extremely heavy), streak, and paramagnetism.
And also go on line or purchase a field guide to rocks and minerals, try to avoid using points or acids in the testing since this might ruin the intrinsic value of the metal itself. Visual identification is the best possible means.
Go to still water mine also know as The Still Water complex in Montana https://en.wikipedia.org/wiki/Stillwater_igneous_complex is the only mine in the us that mines PGMs primarily in the US there are some photos there of Sulfidic serpentintite, platinum-palladium ore but it is also in other rocks like quartz and Dunite .But first have a nickel test done most likely will contain both nickel and copper if found in the US as Platinum / Palladium is a bio product for most Cooper mines along with gold &silver