It has to do with knowing the knowable God.
He who is having my commands, and is keeping them, that one it is who is loving me, and he who is loving me shall be loved by my Father, and I will love him, and will manifest myself to him.
John 14:21
I guess you start by trusting others’ testimonies about God in their lives (including the testimonies from scripture) – that he is a faithful and trustworthy Father, until you have your own undeniable evidence. Then you can be at peace if you have become his child, knowing that he is in control, and absolutely. Maggie, Rich Stearns, Georg Müller, Glenn Morton, yours truly and many many more. The evidence does not have to be objective external third party verifiable as it was in those examples, it can also be internal (but still objective and compelling – it is not just about feelings). Francis Collins, Phil Yancey, and Sy Garte are among those. Just yesterday we learned here of another in @Mervin_Bitikofer’s new topic, René Gerard, in a very cool interview.