How to identify a friend or an enemy ?

त्रुश्चैव हि मित्रं च न लेख्यं न च मातृका ।
यो वै संतापयति यं स शत्रु: प्रोच्यते नृप ॥ – महाभारत २.५५.१०
Meaning : (Duryodhan tells Dhrutrashtra ) O king its neither written nor inscribed on anyone that who is friend of whose and who is an enemy , if someone gets troubled by anyone then the later is an enemy of the former.