The following are some traits to be wary of:
1. The envier is always angry at the decree of Allaah.
2. The envier always complains and rarely thanks Allah even if he owns the whole world.
3. He follows up the mistakes if the one whom he envies, and tries to seek out his faults, and exposes them and exaggerates about them before others.
4. He conceals or ignores or belittles the good qualities and distinguishing characteristics of the person whom he envies.
5. So you will notice that the envier cannot speak in front of the one whom he envies without addressing him in a laughing, jocular manner, but deep down he is filled with hatred and resentment that is clear from the way in which he looks at him.
6. He clearly criticizes the one whom he envies, with or without evidence.
7. He looks for opportunities and makes the most of any chance to harm the one whom he envies in himself or his wealth.
8. Finally, the envier is a troubled man, due to the resentment that is always festering in his heart, so depression and dullness show on his face.
At-Turqul-Hisaan Fi Amradil Jaan
(The Jinn and Human Sickness, by Dr. Abul-Mundhir Khaleel ibn Ibraheem Ameen, pgs. 263-264)