Judges 12:6 Then said they unto him, Say now Shibboleth: and he said Sibboleth: for he could not frame to pronounce it right. Then they took him, and slew him at the passages of Jordan: and there fell at that time of the Ephraimites forty and two thousand.  

Sometimes its as easy as this to determine who enemies are. Simply by listening to how they speak; word usage, inflection, & intonation. If someone claims to be one of faith yet by their own mouth they betray who they really are in their heart.

Our Savior stated in Luke 6: 45 A good man out of the good treasure of his heart bringeth forth that which is good; and an evil man out of the evil treasure of his heart bringeth forth that which is evil: for of the abundance of the heart his mouth speaketh. 

The last part of the verse is the key to understand the concept altogether. Our heart is what directs our words. If our hearts are good or evil its expressed by our mouth.

In Israel the test in the verse mentioned was as simple as a Si• or Sh•. Today we could say its as simple as listening to someone to see if the bless or curse. Sometimes its that easy, sometimes not, because everyone stumbles at some point, that stumble may include a curse but that person could just as easily ask for forgiveness without saying a word.

So listen to words but do not be quick to judge, ours is a bit more complicated than the Si• or Sh•.