What you like?

A man is known by his choice. If you want to judge a person you need to know about likings. It is only thing which a person cannot pretend and a good connoisseur can judge a person. When I talk about likings then it includes everything from his eating habits to his hobby , film liking ,what he njoy doing in his leisure time and his friend circle.
Now the question why do I need to judge a person?
In every phase of ur life u need to trust few people and ur life revolve around them. Whether you want to start friendship or u need ur life partner or you selecting a team member.
Every type of selection requires some specific traits but still good person is good whatever may be the role. What it requires to be likable (not successful because success is always counted by ur fame and money) is not to be intelligentsia but to be better person (by better person I mean with good attitude).

How to decide which are good/bad likings?

This depends upon for which type of role r u going 2 u judge a person. First of you should be clear of ur requirement then only u can judge a person.
For example I can quote an example which I experienced while interacting with HR Head of HCL America. In one of his session he asked about the food liking of different people who came to meet him. He categorized them is three category starting from non-veg to simply veg and eventually concluded that person who prefer simple veg (rice-dal) are easy to pacify and vice versa.
This was just one simple example where he judged people from their likings. Similarly if you feel u are smart enough to judge a person you can use their traits, but caution is that you should be aware about urself.