how do I find out if someone is discrimination against me for a job?

I have worked for my current job for over 4 years. I work part time and I applied for full time. I recently graduated with a BA in English. I work Janitorial and I was told that out of the four people that got a second interview (me being one of them) the three jobs were going to other people that had better qualifications and education. I'm going to find out about these people soon and I believe they didn't give me the job because I got one of the people doing the inter view in trouble for discrimination before.