There is no way to ensure it. You can only beg Allaah Ta`aalaa everyday to keep you firm and steadfast upon Imaan, upon the Haqq, and to grant you Jannah and protect you from Jahannam.
As Imaam Ahmad ibn Hanbal was laying to die, Shaytaan came to him and said, "You're successful. You've beaten me (i.e. I wasn't able to deviate you from the right path)." Imaam Ahmad said, "No. Only when I die with the Kalimah Laa Ilaaha Illallaah will I have beaten you."