What exactly do the results of the Brigance test reveal at age of 5? How can the testers know if a child is serious or pulling their chain when they answer?

Here are two links that explain the Brigance Tests and categories of those test. Reading Assessment Database: Search Results - SEDL Reading Resources

Brigance Diagnostic Inventory of Early Development | Encyclopedia of Childhood and Adolescence | Find Articles at BNET

If this test was performed at a public school, the results of each category should have been explained to you. You have the right to stop the meeting at any point to ask questions. You should have been given a copy of the report with the results explained by category. If you need further explanation of the results, try using 'google' or contact the examiner of the test.

The examiner typically has tested many, many children and usually can tell when a student is not answering to the best of their ability. If a 5 year old is trying to manipulate the test, I would suggest a psychological follow-up.