Purim Katan

When Adar is doubled, Purim can be observed by Noahides on the 14th of Adar to distinguish their practice from the Qehal of Israel's. 

But one thing all believing Jews and Noahides have in common is that Purim is the day we perform the mitzva of eliminating the memory of the Amalekites by using a Gragger. Apostate Jews will talk about actually killing some kinds of human instead. That is how you can distinguish a Believer from an Apostate.