The main way that biases in investigation are prevented are through the chain of custody and the rules of evidence.
The chain of custody means that each person who collects evidence and transports or stores it is responsible for that evidence. There has to be an official record of each person who comes into contact with the evidence, or the chain of custody is broken and the evidence might be inadmissible. This prevents investigators with biases from being able to tamper with evidence. The key in collecting evidence is to make sure it is able to be used in court.
The rules of evidence describe what is admissible and what is not. For example, a person cannot testify that he or she heard someone else say something. This is hearsay. A person also cannot testify if there is a privilege (spousal, religious, or counsel). Evidence that does not follow the chain of custody rules can also be inadmissible. Confessions that were improperly received are not admissible either. This keeps everyone honest and on the same constitutional page.