They are trained to know the characteristics of good visa applicants for both immigrant and non-immigrant visas. Well-qualified applicants generally sail through the process rather quickly. Very poorly qualified applicants are rejected with similar, if not identical, speed.
For example, a well-qualified immigrant visa applicant will have a clear relationship to the person who petitioned for them, and be eligible for the visa for which they have applied. They will be of good health, and either have a sponsor in the country they’re really relocating to or they, will have the financial resources to establish themselves. Often they will already speak some English, and have a motive for relocating that they can enunciate clearly. If the documents are all in order, it’s a quick ‘yes.’ Similarly, a very poor visa applicant for a non-immigrant visa will have no, or a very poor job, limited family ties in their country of origin, no established social ties at home, poor economic prospects at home, and be unable to enunciate the motive to their trip. They are easy ‘no’ cases.