According to Shaykh Ibn 'Uthaymeen رحمه الله, one of the strong signs that a person is inflicted with the disease of hasad is that he always tries to conceal the virtues and goodness of others. He does not like it when others talk about the good that a person has done. He remains silent and pretends like he does not know of the good that the other one has done. 
A true believer who is free of hasad likes it when good things are said about others and when the good deeds of others are appreciated and spoken about. He, himself, when he speaks about others always speaks about their good aspects and the wonderful bounties that Allaah سبحانه و تعالى has bestowed on them. He has no envy in his heart for what Allaah سبحانه و تعالى has bestowed upon the others and, therefore, he spreads that good news to others. 
This sign that Shaykh Ibn 'Uthaymeen رحمه الله has explicitly mentioned should be considered a type of subtle sign that people may not notice in themselves. They must take note of this sign and repel it as quickly as possible as it is a stepping stone to the greater forms of hasad. 
 Ad-Duhaami points out that hasad is one of the greatest causes of backbiting. Not only does the person remain silent as to the others good points, out of envy and hatred, he tries to put down the other person.
 Sharh Riyaadus Saaliheen, vol. 4, p. 707
 Commentary on the Forty Hadeeth of Imaam an-Nawawi رحمه الله, vol. 2, p. 1031