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By the quality of their questions and answers. Some are regular at a particular time and you connect with them at that time even without being friends whereas there are others who come at a time when you dont. But basically if you like the quality of the questions and answers you can go their page and check all their questions and answers and if you feel that you connect with them in the way they think or those who are even superior to you in intellect you can become friends with them and that friendship is fruitful upto question and answers and you can go beyond that if the other side is willing. Finally, one should not be judgemental in the virtual world and what we may perceive as a good person may not necessarily be so in real world.