How does one know if one is a true believer? Well, in reality, one doesn’t. One will only find out if he/she was a true believer on the Day of Judgment.
In this life though, we need to ask Allah to make us true believers. We also need to analyze ourselves and see if our actions resemble those of the believers.
Here’s a good article that explains the difference between a believer and a disbeliever:
The Characteristics of the True Believer
Al-‘Allaamah ‘Abdur-Rahmaan ibn Naasir As-Sa’dee
From the book: Essential Q&As about Eemaan (p.58-63)
[ In the Name of Allaah, the All-Merciful… ]
What are the characteristics of the believers that make them distinct from the disbelievers?
This is a great question indeed. By knowing the difference between the believers and other people, the difference between the Truth and falsehood becomes distinctly clear, as does the difference between the people of eternal happiness and the people of eternal misery.
Know that the true believer is the one who believes in Allaah and all of his Names and Attributes that are found in the Book and the Sunnah, upon their proper understandings. They must be fully acknowledged. Furthermore, the believer exalts Allaah, declaring Him free of anything that contradicts the perfection of His Names and Attributes. Thus, his heart becomes full of Eemaan, knowledge, conviction, serenity, and becomes totally concerned with Allaah.
The believer repents to Allaah alone, performing acts of worship for Him that He has legislated upon the tongue of his Prophet (sallallaahu ‘alayhe wa sallam), with sincerity to Allaah, hoping for His Reward and fearing His Punishment.
The believer is thankful to Allaah in his heart, with his tongue, and with the actions of his limbs. He is thankful for Allaah’s Blessings and the great amount of goodness that Allaah has given him, those that he enjoys at all hours of the day. The believer is thankful and utters phrases of Allaah’s remembrance, and he does not see any blessing greater than what Allaah has given him, nor any token of generosity greater than what has come from Allaah.