Judging is a skill that comes with experience. The more you talk with others, the more you will be able to judge them. So, at first u can concentrate on how to find persons whose profiles "seem" to be convenient for your needs : u may start asking urself: will they be able to help me? can I provide help in return?. later on, you will find out with whom u feel more at ease and you will find it easier to decide whether they are or u r of any help to each other.
I personally have two important rules: 1- Do not rush to make judgements. Give urself and others time to know each other better 2-but just be cautious: do not disclose personal info about urself. Sometimes it can be really dangerous.