When the Talmudic sage Rabbi Chanina ben Dosa prayed on someone's behalf, he would know that his prayer had been accepted if his words flowed fluently, for then he felt his soul to be aligned with his prayers.
But if he felt he was making an effort, as though he were forcing his words against reality, he knew that his prayer had been rejected.
The most complete example of such a prayer is found in the story of Rabbi Chiya and his sons.
When Rabbi Chiya recited the words of prayer, "He causes the wind to blow," the wind blew.
When he said, "He causes the rain to fall," rain fell.
When Rabbi Chiya said, "He brings the dead back to life," the entire world shook.
When a person is connected to his prayer to such a profound extent, he has no need to work at aligning himself with the words of prayer, or with an external framework, simply because that framework is no longer external to him.
A total unity exists between his soul, the words that he is reciting, and objective reality.
–Rabbi Adin Steinsaltz
From The Thirteen Petalled Rose, "Prayer" by Rabbi Adin Steinsaltz