How can a Pope decide someone is a saint or not?

Short answer- Pope can not decide on his own and make someone a Saint.

The Catholic church proclaims a holy preacher to be included in the list or canons of saints after a process called Canonisation

What is Canonisation?

In the year 1234, Pope Gregory IX established procedures to investigate the life of a candidate saint and any attributed miracles. In 1588, Pope Sixtus V entrusted the Congregation of Rites (later named the Congregation for the Causes of the Saints) to oversee the entire process. Beginning with Pope Urban VIII in 1634, various Popes have revised and improved the norms and procedures for canonization.

Thus the process, such as those used today in the Orthodox and Roman Catholic Churches, were developed.

to get the official version click The Process of Beatification and Canonization

The Process of Becoming a Saint

For a lay man it will be difficult to understand. So explaining in simple words.

Part 1

How does someone become a saint?

To answer the question directly, the Catholic Church believes that anyone can become a saint—that is someone who makes it to heaven. Whether you are a priest, a married women, a single man, a religious sister, etc., the Catholic Church calls all men and women, whatever their state in life, to seek holiness and sainthood. This idea has been given more attention recently and maybe most significantly during Vatican II and the released of the document Lumen Gentiumthat outlined what the Catholic Church calls the Universal Call of Holyness.

Why does the Catholic Church choose one person over another to be a saint?

Sometimes we can casually say that the Church is making him or her a saint. But, technically, the Church does not make saints; it recognizes someone who is in heaven. In addition to this, the Church is looking for folks whose lives are worth imitating and to such a degree that they should be held up as an example to the Church. Knowing these criteria can help people understanding the process for how the Catholic Church recognizes saints.

DEVIL’ s ADVOCATES

During the canonization process employed by the Roman Catholic Church, the Promoter of the Faith (Latin: promotor fidei), popularly known as the Devil's advocate (Latin: advocatus diaboli), was a canon lawyer appointed by Church authorities to argue against the canonization of a candidate.

It was this person’s job to take a skeptical view of the candidate's character, to look for holes in the evidence, to argue that any miracles attributed to the candidate were fraudulent, and so on. The Devil's advocate opposed God's advocate (Latin: advocatus Dei; also known as the Promoter of the Cause), whose task was to make the argument in favor of canonization. During the investigation of a cause, this task is now performed by the Promoter of Justice (promotor iustitiae), who is in charge of examining the accuracy of the inquiry on the saintliness of the candidate.

What is the process of being recognized as a saint in the Catholic Church?

This is probably the question most people are really asking when they are asking: How does someone become a saint? Just this week it was announced that Pope John Paul II would be canonized a saint (along with John XXIII). But, similar news about John Paul II becoming a saint was released about two months ago. Why was this news announced twice? There were two announcements (maybe more) because people who become canonized saints go through several processes to become a saint and announcements about the potential saint are released after each one.

5 Steps to Sainthood

First, the person’s local bishop investigates their life by gathering information from witnesses of their life and any writings they may have written. If the bishop finds them to be worthy of being a saint, then he submits the information that he gathered to the Vatican’s Congregation for the Causes of Saints.

Second, the Congregation for the Causes of Saints can choose reject the application or accept it and begin their own investigation of the person’s life.

Third, if the Congregation for the Causes of Saints approves of the candidate, they can choose to declare that the person lived a life heroically virtuous life. This isn’t a declaration that the person is in heaven, but that they pursued holiness while here on earth.

Fourth, to be recognized as someone in heaven requires that a miracle has taken place through the intercession of that person. The miracle is usually a healing. The healing has to be instantaneous, permanent, and complete while also being scientifically unexplainable. Miracles have to be first verified as scientifically unexplainable by a group of independent doctors, then the person is approved by a panel of theologians, and then the final approval lies with the pope. If this is the case, a person is declared a blessed.

Fifth, a second miracle is needed in order to declare someone a saint. The confirmation of a second miracle goes through the same scrutiny as the first.