It is not important the length of the prayer, or the selection of it's words, but the depths of feelings in it...

One sentence a man says in depth, Like "Lord have mercy", so one of the twenty four elders comes forward and takes this prayer in his golden censor and ascends with it to the throne of God as a smell of inscence which are the prayers of the saints.

HOW CAN WE DISTINGUISH THE DEEP PRAYER?

It is a prayer which has a feeling of relation with God, a prayer with passion, understanding, contemplation, warmth, love, humility, contrition ..... with faith, confidence, and hope...