Thank you for using the Jiskha Homework Help Forum. Here is an answer another student gave last year for the same question:
Some sociologists use what is called the Likert Scale after asking a series of open-ended questions
Another student answer: I found the answer. There is no scientific way to calculate prejudice. Bogardus scale was one way though.
(Spark Notes): http://www.sparknotes.com/psychology/psych101/socialpsychology/section2.rhtml
P.S. Here's a wonderful paper on the subject from Harvard University. It's a PDF file to download: