Paul brought up the idea of testing ourselves to be sure we are “in the faith.” John was very clear in his statement in what we call his first epistle: “We know that we have passed from death to life because we love our brothers. The one who does not love remains in death.”
(1 John 3:14,15 Holman Christian Standard Bible®©)
It is easier to tell if children get saved than it is when adults do, sometimes. The easiest way to check it out is to observe the new lives for the fruit of the Spirit (Galatians 5:22,23).
Children become willing servants when they become believers. Adults should, but they have usually built up enough pride to keep them from showing their salvation with the evidence of becoming humble and serving people. Adults who are already humble, and those who know that they had to be forgiven much are usually exceptions that are easy to spot.