Ways you can know god is:
The final, complete, definitive way, of course, is Christ, God himself in human flesh.
His church is his body, so we know God also through the church.
The Scriptures are the church's book. This book, like Christ himself, is called "The Word of God."
Scripture also says we can know God in nature see Romans 1. This is an innate, spontaneous, natural knowledge. I think no one who lives by the sea, or by a little river, can be an atheist.
Art also reveals God. I know three ex-atheists who say, "There is the music of Bach, therefore there must be a God." This too is immediate.
Conscience is the voice of God. It speaks absolutely, with no ifs, ands, or buts. This too is immediate. [The last three ways of knowing God (4-6) are natural, while the first three are supernatural. The last three reveal three attributes of God, the three things the human spirit wants most: truth, beauty, and goodness. God has filled his creation with these three things. Here are six more ways in which we can and do know God.]
Reason, reflecting on nature, art, or conscience, can know God by good philosophical arguments.
Experience, life, your story, can also reveal God. You can see the hand of Providence there.
The collective experience of the race, embodied in history and tradition, expressed in literature, also reveals God. You can know God through others' stories, through great literature.
The saints reveal God. They are advertisements, mirrors, little Christs. They are perhaps the most effective of all means of convincing and converting people.
Our ordinary daily experience of doing God's will will reveal God. God becomes clearer to see when the eye of the heart is purified: "Blessed are the pure of heart, for they shall see God."
Prayer meets God—ordinary prayer. You learn more of God from a few minutes of prayerful repentance than through a lifetime in a library.
And for the other question the answer is: accept as true; feel sure of the truth of.
