The hair styles of Graves and Grindelwald are similar.
In the interrogation, Graves pays particular attention to the fact that Albus Dumbledore holds Newt Scamander in high regard. According to Harry Potter and the Half-Blood Prince, Dumbledore and Grindelwald were close friends, possibly lovers, and equally obsessed with the Deathly Hallows.
It is later revealed that the point-of-view shots of someone gliding through the city is Graves travelling as smoke. Travelling as smoke in the films is associated with the dark arts, as the only other people that have been seen traveling in this way in this fictional universe are Death Eaters and Voldemort.
In one secret meeting between Graves and Credence Barebones, Graves gifts him a charm which can summon him, should Credence need help. The charm is the symbol of the Deathly Hallows. In the Harry Potter books it is also understood to be the mark Grindelwald once identified with in his terrorism campaign in the 1920s.
Dumbledore's sister Ariana is rumored to host an obscurus, which is why in the books she was prone to lash out with strong magic after attempting to suppress it. Save Scamander, Graves appears to be the only one knowledgeable about obscurials in this film - at least enough to think he could use Credence for his nefarious ends.
Believing Credence to be non-magical with magical lineage, Graves refers to him as a squib. Given that the magic community refers to non-magical people without magical lineage as No-Majs instead of muggle, it is entirely possible that they have another term for squibs. If so, Graves-Grindelwald slipped up and exposed his European origins.
Graves' first name is Percival. Percival is one of Albus Percival Wulfrick Brian Dumbledore's middle names. Grindelwald might have, consciously or unconsciously, recalled 'Percival' and used it for his alias.