It is our belief that we cannot confirm 100% if someone is going to jannah or not.  The Prophet (pbuh) mentioned in many hadith, signs that one is accepted into jannah.

A janazah passed by the sahabah and they started to praise the deceased. The Prophet (pbuh) said “It became certain, it became certain”. Then another janazah passed by and the they stared to condemn the deceased. The Prophet (pbuh) said that the first one you praised, jannah has become mandatory for him because of your praises, while the second one, hell fire is for him because of your condemnation. 

May Allah (swt) give us all jannat ul firdous.