Determining bias is not 100% objective. Obviously, I have my own biases, as do you. In determining the biases of media on this site several factors were taken into consideration. First and foremost was the research conducted by credible sources, see list below. Next was a 5 year personal study of loaded words, misleading headlines and most importantly choice in reporting. For example, a right or left wing source may report something terrible that either side did, but they will not report it if it is negative and directed at their side. Most centrist sources would use language that allows the reader to decide rather than assisting them with loaded words. To put it simply, bias in media, is the media telling you what to think about information. There is nothing wrong with that as long as you are aware that it is occurring. Truthful information can be found in both Right and Left wing sources, but the inference of meaning is where you, the reader, need to formulate your own interpretation. The purpose of this site is to help people determine when and where bias is occurring, so they can find the truth, and interpret the information themselves, without relying on the bias of media.
There are many more I did not note, but this is a start.