I've seen a lot of soulmate fics for this pairing, but none with the traditional name-on-wrist variant. So, all his life, Newt's had 'Percival Graves' on his wrist. The polyjuice potion doesn't copy soulmarks, and the connection that happens when soulmates touch doesn't transfer either. So, when Newt turns up in New York, he knows almost the moment Grindlegraves touches him that something is seriously wrong.