God and Saint are like magnet. Magnet attracts pure steel. Place a magnet in the middle and place needles around it. The more purity the needle has, the more quickly it will be attracted to the magnet; the more impurity the needle has, the longer it will take to be attracted to the magnet.
The more impure the heart is, the longer it will take to be drawn to God and Saint. For example, if there are four people in a family -- father, son, mother, brother or anyone for that matter; all four go to the satsang of a Saint. Whoever's heart is most pure, the sooner he would be drawn to the Saint, would surrender. And whoever's heart is impure, the longer it will take to be drawn to the Saint.
Even to the extent that Bhagwan Ram, Who's Divine beauty enticed the hearts of Saraswati and Brihaspati, and even Parvati and Laxmi, Who are beyond Saraswati and Brihaspati there, such people were standing who thought that Bhagwan Ram looked scary: "He has eighteen noses, twenty-two ears, big teeth." In other words, Ram appeared scary to those who had an impure heart; those people were scared: "We are scared to look at Him." One Who is the personification of Divine blissfulness, Whose Divine beauty makes uncountable Kamdev (god of material beauty) faint, appeared scary to those with impure hearts.
So the proof of a pure is heart is that it would be attracted upon coming into contact with something pure. How soon it becomes attracted depends on how pure it is and how impure it is. There is no other thermometer to measure the extent of purity.